Glass, optical glass, glass raw material for press molding, and optical element

ABSTRACT

A glass including at least one type of oxide selected from TiO 2 , Nb 2 O 5 , WO 3 , and Bi 2 O 3  as a glass component therefor, having a total TiO 2 , Nb 2 O 5 , WO 3 , and Bi 2 O 3  content of at least 20 mol %, and having a βOH value indicated in formula (1) that fulfills the relationship indicated in formula (2). 
       βOH=−[ln( B/A )]/ t   (1);
 
       βOH≧0.4891×1 n (1/ HR )+2.48  (2).

TECHNICAL FIELD

The present invention relates to a production method of glass, opticalglass, press-molding glass and optical element having excellenttransmittance.

DESCRIPTION OF THE RELATED ART

Recently, as devices of photographic optical system and projectionoptical system or so has become more functional and more compact, thereare increasing needs of an optical glass having high refractive index asthe material of effective optical element.

The optical glass having high refractive index comprises large amount ofhigh refractive index component such as Ti, Nb, W, Bi or so as the glasscomponent. These components are easily reduced during the meltingprocess of the glass, and these components being reduced absorbs thelight of the short wave length side at the visible light range; thus thecoloring of the glass increased (hereinafter, it may be referred as“reduced color”).

Also, the high refractive index components which are easily reducedreacts (oxidizes) with noble metal material such as platinum or so whichare widely used as the material of the crucible; and the noble metal ionproduced by the oxidation of the noble metal causes to dissolve in themolten glass. The noble metal ion dissolved in the molten glass absorbsthe visible light; hence the coloring of the glass increases.

The optical glass having high refractive index comprising a lot of highrefractive index component had problems such as the coloring of theglass, and particularly the transmittance of the short wave length sideat the visible light range easily decreased. As the means to solve suchproblem, the patent article 1 proposes the technical arts to bubble thenon-oxidizing gas in the molten glass, or the technical art of heattreating the obtained glass by re-heating it.

However, when melting the glass comprising large amount of highrefractive index component such as Ti, Nb, W, Bi or so, when bubblingthe reducing gas such as carbon monoxide, hydrogen or so which arerecited in the patent article 1, the high refractive index componentadded as the oxidized product is reduced and becomes metal, forms alloywith the metal material such as platinum or so constituting the meltingcontainer; thus the strength and the durability of the melting containerdeclined significantly. Also, as the inactive gas such as helium orargon or so are expensive, these are not suitable for the bubblingtaking long time as the production cost will increase.

Also, since the melting of the glass is generally carried out in the airatmosphere, the oxygen in the air may react with the noble metalmaterial such as platinum or so which is the material of the meltingcontainer. Particularly, in case the melting container is platinum basedmaterial, platinum dioxide (PtO₂) is generated and dissolves into themolten material; or it may dissolve into the molten material as platinumion (Pt⁴⁺) from the boundary between the molten material and theplatinum based material. As a result, coloring of the glass may occur.

Therefore, the technical art to bubble the non-oxidized gas as in thepatent article 1 cannot sufficiently suppress the noble metal such asplatinum or so from dissolving into the glass, thus it was stilldifficult to significantly reduce the coloring of the optical glasshaving the high refractive index.

PRIOR ART Patent Article

[Patent Article 1] Japanese Patent Application Laid Open. No.2011-246344

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

The present invention was achieved in view of such circumstances, andits object is to provide the glass, optical glass, press-molding glassand optical element having excellent transmittance.

Means for Solving the Problems

As a result of the keen examination in order to achieve the abovementioned object, the present inventors have found that by controllingthe value of βOH of the glass, and the total amount (mol %) of thecontent of each component of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ included in theglass (hereinafter, it may be simply referred as “the content of thehigh refractive index component”) to satisfy the predeterminedrelationship, the object thereof can be attained; and by such findingthe present invention has been achieved.

The gist of the present invention wherein the object is to solve suchproblem is as described in below.

[1] A glass comprising at least one oxide selected from the groupconsisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as a glass component, wherein

a total content of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is 20 mol % or more,and

a value of βOH shown in below equation (1) satisfies a relation shown inbelow equation (2).

βOH=−[ln(B/A)]/t  (1)

βOH≧0.4891×ln(1/HR)+2.48  (2)

[In the equation (1), “t” is a thickness of said glass used for ameasurement of an external transmittance, “A” is the externaltransmittance (%) at a wavelength of 2500 nm when a light enters intosaid glass in parallel to a thickness direction thereof, and “B” is sthe external transmittance (%) at the wavelength of 2900 nm when a lightenters into said glass in parallel to the thickness direction thereof.In the equation (2), “HR” shows a total amount (mol %) of a content ofeach component of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ in said glass. In theequations (1) and (2), “ln” is natural logarithm.][2] The glass as set forth in [1], wherein a content of noble metal is 4ppm or less.[3] The glass as set forth in [1] or [2] comprising P₂O₅ as said glasscomponent.[4] An optical glass comprising the glass as set forth in any one of [1]to [3].[5] A glass material for press-molding comprising the optical glass asset forth in [4].[6] An optical element comprising the optical glass as set forth in [4].

THE EFFECT OF THE INVENTION

According to the glass production method of the present invention, thetransmittance of the glass can be improved drastically, by controllingthe value of βOH of the glass, and the content of the high refractiveindex component to satisfy the predetermined relationship. Also, theamount of the noble metals such as platinum or so dissolved into theglass can be reduced significantly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the flow chart of steps from the preparation of the batchraw material to the production of the glass.

FIG. 2 is a graph showing the relation between βOH and the highrefractive index component (HR) of the sample according to theembodiment of the present invention.

FIG. 3 Regarding the second modified example of the present invention,FIG. 3 is a graph showing the change of the external transmittance(T450) at the wavelength of 450 nm when the light enters parallel to thethickness direction of the No. 1 glass having the thickness of 5 mm withrespect to βOH value when βOH value of the No. 1 glass is changed fromthe composition of the Table 1.

FIG. 4 Regarding the third modified example of the present invention,FIG. 2 is a graph showing the change of the external transmittance(T450) at the wavelength of 450 nm when the light enters parallel to thethickness direction of the No. 3 glass having the thickness of 5 mm withrespect to βOH value when βOH value of the No. 3 glass is changed fromthe composition of the Table 2.

FIG. 5 is a graph showing the relation between βOH and the refractiveindex of the sample according to the first modified example of thepresent invention.

THE EMBODIMENTS FOR CARRYING OUT THE INVENTION Glass

The glass according to the present invention is the glass including atleast one oxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃and Bi₂O₃ (hereinafter, it may be simply referred as “the content of thehigh refractive index component”) as a glass component, wherein a totalcontent of said TiO₂, Nb₂O₅, WO₃, and Bi₂O₃ is 20 mol % or more, and avalue of βOH shown in below equation (1) satisfies a relation shown inbelow equation (2).

βOH=−[ln(B/A)]/t  (1)

βOH≧0.4891×ln(1/HR)+2.48  (2)

Here, in the above equation (1), “t” is the thickness (mm) of said glassused for the measurement of the external transmittance, “A” is theexternal transmittance (%) at the wavelength of 2500 nm when the lightenter into said glass in parallel to the thickness direction thereof,and “B” is the external transmittance (%) at the wavelength of 2900 nmwhen the light enter into said glass in parallel to the thicknessdirection thereof. Also, in the above equations (1) and (2), “ln” is anatural logarithm. The unit of βOH is mm⁻¹.

Note that, “external transmittance” is the ratio (lout/lin) of theintensity “lout” of the transmitted light which transmitting out theglass with respect the intensity “lin” of incident light which entersinto the glass, that is the transmittance which considers the surfacereflection at the glass surface as well; and “internal transmittance”which will be described in below refers to the transmittance in casethere is no surface reflection at the glass surface (that is, thetransmittance of the glass itself constituting the glass). Eachtransmittance can be obtained by measuring the transmission spectrumusing the spectrophotometer.

Also, in the above mentioned equation (2), HR shows the total amount(mol %) of the content of each component TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ insaid glass. In order to obtain the high refractive index glass, thetotal content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is 20 mol % or more, that isthe value of HR is 20 or more. Preferably, the lower limit of HR is 25,more preferably 30, and further preferably 35. Also, the upper limit ofHR is preferably 85, more preferably 80, and further preferably 75.

Also, for the glass according to the present embodiment, the value ofβOH shown in the above equation (1) preferably satisfy the relationshown in the below equation (3), more preferably satisfy the relationshown in below equation (4), and further preferably satisfy the relationshown in below equation (5).

βOH≧0.4891×ln(1/HR)+2.50  (3)

βOH≧0.4891×ln(1/HR)+2.53  (4)

βOH≧0.4891×ln(1/HR)+2.58  (5)

Also, the upper limit of βOH differs depending on the type and theproduction condition of the glass, and as long as it can be adjusted, itis not particularly limited. If βOH is increased, the amount of thevolatile product from the molten glass tends to increase, hence from thepoint of suppressing the volatilization from the molten glass, βOH is 10mm⁻¹ or less, preferably 8 mm⁻¹ or less, more preferably 6 mm⁻¹ or less,even preferably 5 mm⁻¹ or less, even further preferably 4 mm⁻¹ or less,still more preferably 3 mm⁻¹ or less, and still even further preferably2 mm⁻¹ or less.

βOH shown in the above equation (1) refers to the absorbance byhydroxide group. Therefore, by evaluating βOH, the concentration ofwater (and/or the hydroxide ion, hereinafter simple “the water”)included in the glass can be evaluated. That is, the glass having highβOH means the water concentration included in the glass is high.

In the glass according to the present embodiment, the value of βOHsatisfies the relation shown in the above equation (2). That is, theglass according to the present embodiment is controlled so that thewater concentration in the glass to be higher than a certain value.

The method to make βOH higher in the glass is not particularly limited,however the procedure to increase the water content in the molten glassduring the melting step may be mentioned. Here, as the procedure toincrease the water content in the molten glass, for example thetreatment to add the water vapor in the melting atmosphere, and thetreatment of bubbling the gas including the water vapor into the moltenmaterial or so may be mentioned.

Usually, according to these methods, the water can be introduced intoglass, and βOH can be increased, however the increasing rate thereofdiffers depending on the glass composition. This is because the easinessto take in the water to the glass differs depending on the glasscomposition.

In case the glass composition is those which take in the water easily,by carrying out the treatment to increase βOH as mentioned in the above,βOH of the glass can be increased significantly. However, in case theglass composition is those which barely takes in the water, even if thetreatment is carried out in the same condition, it is difficult toincrease the value of βOH to the same level as the glass compositionwhich takes in the water easily, thus βOH of obtained glass becomes low.

Also, in case the glass composition is those which take in the water ineasily, even if it is the glass produced by the usual production method,it actively take to the water in the melting atmosphere (the airatmosphere), hence the value of βOH becomes higher than the glasscomposition which barely takes in the water.

As such, the easiness to take in the water to the glass differsdepending on the glass composition. Thus, in the present invention, theabove mentioned equation (2) is defined based on the difference ofeasiness to take in the water by the composition, and the lower limit ofβOH depending on the glass composition determined.

Here, in the above equation (2), HR is the total amount (mol %) of thecontent of each component TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ in the 100 mol % ofsaid glass.

As discussed in the above, depending on the glass composition, there isa glass wherein the water can be taken in easily and that βOH can beincreased easily, and those glass which are not. As a result of the keenexamination, the present inventors have found a tendency that the higherthe ratio of each component of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ in the glasscomposition is, the easier the water is taken in, thereby the abovementioned equation (2) was determined.

The above mentioned equation (2) as such separates whether or not theglass has been carried out with the treatment to increase βOH during theproduction steps thereof. That is, during the production steps of theglass, the glass which did not carry out the treatment to increase βOH(the glass produced based on the conventional production method) doesnot satisfy the above mentioned equation (2).

Nevertheless, for the glass comprising large amount of the highrefractive index component such as Ti, Nb, W, Bi or so as the glasscomponent, usually the high refractive index components are reducedduring the melting step of the glass, and absorbs the light at the shortwavelength side of the visible light range, thus there was a problemthat the coloring is increased in the obtained glass.

The coloring of such glass (hereinafter, it may be referred as thereduced color) is reduced by carrying out the re-heat treatment to theglass under the oxidizing atmosphere. This is thought to be caused aseach ion such as Ti, Nb, W, Bi or so under the reduced state areoxidized by carrying out the re-heat treatment under the oxidizingatmosphere, thereby the visible light absorbance of each ion isweakened.

Particularly, in order to reduce the coloring in short period of time,it is necessary to make the oxidation speed of Ti, Nb, W, Bi or sofaster during the heat treatment, and in order to do so, it is necessaryto have ions which can oxidize Ti, Nb, W, Bi or so by moving inside theglass in a speedy manner and give the electric charge. As for such ion,H⁺ is thought to be suitable.

Here, the glass according to the present embodiment satisfies the abovementioned equation (2). That is, it means that the sufficient water isintroduced in the glass, and large amount of H⁺ derived from water ispresent. As a result, due to the re-heat treatment, H⁺ moves inside theglass in a speedy manner to give the electric charge, and each ion ofTi, Nb, W, Bi or so can be efficiently oxidized. Thereby, in the glassaccording to the present embodiment, the coloring can be significantlyreduced by heat treatment of short period of time, and the glass ofafter the re-heat treatment has an excellent transmittance.

Note that, the infrared light transmit through even the glass with darkcolor, hence βOH can be evaluated regardless of the presence of thecoloring (the presence of the reduced color) of the glass. Also,usually, the re-heat treatment is carried out at the temperature lowerthan the softening point of the glass, and βOH value of the glass beforeand after thereof does not substantially change, thus it can be measuredat any time before and after the re-heat treatment. Therefore, βOH ofthe glass can be measured from either of the transparent glass which hasgone through the re-heat treatment (the treatment for reducing thecolor), and the glass with dark color which has not gone through there-heat treatment.

The glass of the present embodiment is not particularly limited, as longas the above mentioned equation (2) is satisfied, it may be carried outwith the re-heat treatment (the treatment to decrease the reducedcolor), or it may not be carried out with this treatment.

Also, the glass according to the present embodiment has lesser dissolvedamount of the noble metal such as platinum or so which is used as themelting container material or the melting apparatus material of theglass. That is, the glass according to the present embodiment has verylittle amount of the content of the noble metal thereof even in case ofincluding the noble metal.

From the point of reducing the coloring of the glass caused by the noblemetal ion, improving the transmittance, reducing the solarization, andreducing the noble metal contaminant or so, the content of the noblemetal in the obtained glass is 4 ppm or less. The lower the upper limitof the content of the noble metal is, the more preferable it is, and itis further preferable to have lower upper limit in the order of 3 ppm,2.7 ppm, 2.5 ppm, 2.2 ppm, 2.0 ppm, 1.8 ppm, 1.6 ppm, 1.4 ppm, 1.2 ppm,1.1 ppm, 1.0 ppm, 0.9 ppm. The lower limit of the content of the noblemetal is not particularly limited; however 0.001 ppm or so will beincluded inevitably.

As the noble metal, a metal simple substances such as Pt, Au, Rh, Ir orso, and alloy such as Pt alloy, Au alloy, Rh alloy, Ir alloy or so maybe mentioned. As for the melting container material or the meltingapparatus material, Pt or Pt alloy is preferable as it has heatresistance and corrosion resistance among the noble metals. Therefore,for the glass produced using the melting container and melting apparatusmade of Pt or Pt alloy, the content of Pt comprised in the glass ispreferably 4 ppm or less. As for more preferable upper limit of thecontent of Pt, it is the same as the content of the noble metal includedin the glass. Also, the lower limit of the content of Pt is notparticularly limited; however 0.001 ppm or so will be includedinevitably.

In the below explanation, the example using Pt for the melting containerwill be used, however it is the same for the case using the noble metalsother than Pt as the melting container or so.

The glass according to the present embodiment carries out the procedureto increase the water content in the molten glass during the productionsteps thereof. Therefore, the oxygen partial pressure in the meltingatmosphere is reduced, and the oxidation of the noble metal such asplatinum or so which is the material of the melting container (thecrucible or so) is prevented. As a result, the oxygen in the meltingatmosphere reacts with the platinum material or so, and generatedplatinum dioxide or the platinum ion (Pt⁴⁺) is effectively preventedfrom dissolving; thus the dissolved amount of Pt is reduced in theobtained glass.

Usually, the noble metal ion dissolved in the molten glass absorbs thevisible light, hence it has a problem that the coloring increases.However, the glass according to the present embodiment has sufficientlyreduced content of Pt as mentioned in the above, thus the coloringcaused by Pt ion is less and has excellent transmittance.

Also, the glass according to the present embodiment has excellenttransparency. In the production step of the glass (particularly in themelting step), by carrying out the procedure to increase the watercontent in the molten glass, it is thought that the dissolved gas in themolten gas can be increased. As a result, in the glass according to thepresent embodiment, due to the excellent transparency, the time requiredfor the refining step can be shortened in the production steps thereof,thus the productivity improves.

The glass according to the present embodiment can be suitably used asthe optical glass.

Usually, the optical glass of the high refractive index comprises largeamount of the high refractive index components such as Ti, Nb, W, Bi orso as the glass component, thus the coloring (the reduced color) of theglass is demanded to be reduced as mentioned in the above.

The optical glass of the present embodiment can efficiently remove thereduced color by the re-heat treatment even in case of comprising largeamount of the high refractive index components as mentioned in above.Also, the optical glass of the present embodiment is drastically reducedwith the content of Pt, hence there is only little coloring caused byPt. Such optical glass according to the present embodiment has highrefractive index while having excellent transmittance.

The Production Method of the Glass

Next, as the glass according to the present embodiment, one example ofthe production method will be explained by referring to FIG. 1 and usingthe production method of the optical glass as one example.

The production method of the optical glass according to the presentembodiment preferably comprises the rough melting step P1 of obtainingthe cullet 1 by melting the mixed material, and the re-melting step P2of obtaining the glass 2 by re-melting said cullet 1; and

the procedure to increase the water content in the molten glass iscarried out in at least one of said rough melting step or saidre-melting step.

Here, the procedure to increase the water content in the molten glass isnot particularly limited; however it is preferable to carry out at leastthe treatment to add the water vapor in the melting atmosphere and thetreatment of carrying out the bubbling of the gas comprising the watervapor in the molten material.

Hereinbelow, in accordance with FIG. 1, the example of carrying out thetreatment to add the water vapor to the melting atmosphere at both ofthe rough melting step P1 and the re-melting step P2 will be shown;however the treatment to add the water vapor to the melting atmospheremay not be carried out at either one of the rough melting step P1 andthe re-melting step P2.

Note that, if the procedure to increase the water content in the moltenglass is not carried out, and the glass is maintained in the meltingcondition, the water content in the molten glass gradually decreases.Therefore, in order to increase βOH of the obtained glass by solidifyingthe molten glass, it is preferable to carry out the procedure toincrease the water content in the molten glass at the later stage of theglass production step that is at the re-melting step P2, particularly atthe later step of the re-melting step P2, that is it is preferable tocarry out the procedure to increase the water content in the moltenglass at the step of uniforming the molten glass.

Further, as the procedure to increase the water content in the moltenglass, in addition to the treatment to add the water vapor in themelting atmosphere, or instead of this treatment, the treatment ofbubbling the gas including the water vapor in the molten material may becarried out at either one of or at the both of the rough melting step P1and re-melting P2.

[The Rough Melting Step P1]

The rough melting step is the step of obtaining the cullet 1 by meltingthe mixed material.

The rough melting step according to the present embodiment preferablycomprises the step s1 of preparing the batch raw material by mixing theraw material, the step s2 of heating and melting said batch rawmaterial, and the step s3 of obtaining the cullet 1 by cooling themolten material.

(The Step S1 of Preparing the Batch Raw Material)

First, the glass raw material, the mixed raw material (the batch rawmaterial) was obtained by scaling and thoroughly mixing the raw materialcorresponding to the glass component.

As the mixing method, it is not particularly limited, and the knownmethods can be used. For example, the mixing by using the ball mill orthe dry mixer can be mentioned.

As the raw material corresponding to the glass component, it can besuitably selected depending on the glass composition; however oxide rawmaterial, carbonate raw material, nitrate raw material, phosphoric acidraw material, and phosphate raw material or so may be mentioned.

(The Step S2 of Heating and Melting the Batch Raw Material)

Next, the mixed material is placed inside the rough melting container,and then it is heated and melted.

The container and the apparatus used for the rough melting can besuitably selected depending on the composition of the glass to beproduced, and for example the container or the apparatus made of noblemetal (for example, made of platinum or platinum alloy) or quartz may beused.

For example, in case of phosphate glass including P₂O₅, and at least oneoxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃,that is comprising the high refractive index component, during theproduction steps, the melting product material showing significanterosion is produced when the batch raw material is heated and melted.Such melting product material tends to erode the material havingexcellent corrosion resistance such as platinum or so. Such meltingproduct material tends to erode the material having excellent erosionresistance such as platinum or so. Hence, the noble metal materials suchas platinum or so is eroded by the above mentioned melting productmaterial, and dissolve into the molten material and generate contaminantor increase the coloring of the glass.

On the contrary, the flame resistant product such as quartz or so iseroded by the above mentioned melting product material, however even ifit gets into the molten material by being eroded, it becomes part of theglass composition; hence it has lesser problem such as in case of noblemetal material. Therefore, in case of producing the phosphate glasscomprising the high refractive index component, the container and theapparatus used for the rough melting is preferably the container and theapparatus of flame resistant such as quarts or so.

On the other hand, in case of the glass including B₂O₃ and at least oneoxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃,that is in case of borate glass comprising the high refractive indexcomponent, there is less the problem such as the erosion of the noblemetal by the molten product material such as in the above mentionedphosphate glass. Therefore, in case of producing the borate glasscomprising the above mentioned high refractive index components, thecontainer and the apparatus used for the rough melting is preferably thecontainer or the apparatus made of noble metal such as platinum orplatinum alloy which are hardly eroded during the production process ofthe glass. Note that, in case of the borate glass, the flame resistantcontainer such as quartz or so tends to be eroded significantly.

The melting temperature (rough melting temperature) of the batch rawmaterial during the rough melting is preferably within the range of 800to 1400° C. Note that the solubility of the dissolved gas declines asthe temperature of the molten material increases, hence for increasingthe refining effect, the temperature of the molten material during therough melting step is preferably the same as the melting temperature(the re-melting temperature) of the cullet during the re-melting step,or it is preferably less than the melting temperature of the cullet; andparticularly it is preferably lower than the refining temperature duringthe re-melting step.

Also, the melting time during the rough melting step can be adjustedappropriately by considering the amount introduced into the crucible ofthe batch raw material and the capacity of crucible, and for example themelting time may be within the range of 0.1 to 20 hours.

The melting atmosphere of the rough melting step is not particularlylimited; however from the point of increasing βOH of the glass obtainedat the end, it is preferable to add the water vapor to the meltingatmosphere.

By adding the water vapor to the melting atmosphere, the value of βOH ofthe optical glass obtained at the end and the content of the highrefractive index component can be regulated to satisfy the predeterminedrelationship, and also even in case the melting is carried out using theplatinum container or the platinum alloy container, the dissolving of Ptor so to the glass can be effectively prevented, and further thedissolved gas can be supplied to the glass sufficient enough to improvethe transparency.

The method to add the water vapor to the melting atmosphere is notparticularly limited, The method of adding the water vapor in meltingatmosphere is not particularly limited, but for example the method ofintroducing the connecting pipe to the crucible from the opening partprovided at the melting device, and depending on the needs, supplyingthe gas comprising the water vapor through this pipe to the space in thecrucible may be mentioned.

The melting of the rough melting step can be carried out with thebubbling in order to make the molten material uniform. The bubblingduring the rough melting may be continued after the mixed material hasbeen melted. Also, for making the molten material uniform, the moltenmaterial may be stirred by other method than the bubbling.

Note that, the rough melting step is the step to produce the culletwhich is the intermediate product; hence it is not a must to make themolten material uniform. The method of making uniform may be selectedfrom the known method suitably depending on the embodiment of the roughmelting step.

Also, the gas used for the bubbling is not necessarily limited, and theknown gas can be used, and commercially available ones or the onesproduced can be used as well.

The gas used for the bubbling is preferably a gas including the watervapor, from the point that the value of βOH of the optical glassobtained at the end and the content of the high refractive indexcomponent can be regulated to satisfy the predetermined relationship,and also even in case the melting is carried out using the platinumcontainer or platinum alloy container, the dissolving of Pt or so to theglass can be effectively prevented. Further, from the point that thedissolved gas can be supplied to the glass sufficient enough to improvethe transparency; the gas used for the bubbling is preferably the gasincluding the water vapor.

The content of the water vapor in the gas including water vapor as suchis preferably 10 vol % or more, more preferably 20 vol % or more,further preferably 30 vol % or more, even more preferably 40 vol % ormore, even further preferably 50 vol %, furthermore preferably 60 vol %,even furthermore preferably 70 vol % or more, particularly preferably 80vol % or more, and further particularly preferably 90 vol % or more. Thehigher the content of the water vapor is, the more preferable it is; andparticularly by having it within the above mentioned range, the value ofβOH of the optical glass obtained at the end and the content of the highrefractive index component can be regulated to satisfy the predeterminedrelationship.

(The Step S3 of Producing the Cullet)

Next, the molten material is rapidly cooled and the cullet is produced.

The method of rapid cooling of the molten material is not particularlylimited, and the known methods can be used; for example the method offorming the cullet by dropping the molten material into the water andcool, then solidifying; the method of draining the molten material on tothe heat resistant plate, then cooling the molten material andsolidifying it followed by pulverizing to produce the cullet or so maybe mentioned.

The cullet is made of glass; however it does not have to be a uniformglass. Also, the cullet may comprise bubble. Further, the non-meltingmaterial of the batch raw material may be included as well. For thecomposition and the optical characteristic (for example, the refractiveindex and Abbe number or so) of the cullet, the glass of uniform andwithout bubble is formed by re-melting the cullet, and the compositionand the optical characteristic of this glass are defined as thecomposition and the optical characteristic of cullet respectively.

The size of the cutlet can be adjusted suitably considering the storageor transportation, or the easiness to handle in the subsequent steps.For example, in case of producing it by dropping the molten materialinto the water, the dropping amount can be adjusted to control the size.Also, in case of producing it by draining the molten material on themetal plate, the obtained glass can be pulverized to a suitable sizethereby it can be adjusted.

Note that, from the point of preventing the separation, the bubbling canbe continued while the molten material is draining out from the roughmelting container. Further, from the point of increasing the dissolvedgas in the cullet and also from the point of increasing βOH of theobtained glass, the bubbling is preferably carried out by the gasincluding the water vapor.

(The Refractive Index Measurement of the Cullet S4)

Along with the draining of the molten material, a part of the moltenmaterial is taken from the rough melting container for molding and useas the glass sample for the refractive index measurement. Then, therefractive index of this glass sample is measured, and the obtainedrefractive index is defined as the refractive index of the cullet.

The refractive index measurement of the cullet is not necessarilyessential step; however by going through such step, it is preferablesince the characteristic of the optical glass can be regulatedaccurately.

[The Re-Melting Step P2]

The rough melting step is the step of obtaining the optical glass 2 byre-melting the cullet 1.

The re-melting step according to the present embodiment comprisespreferably the step s5 of mixing said cullet 1, the step s6 of heatingand melting said cullet 1, the step s7 of refining the molten glass, thestep s8 of uniforming the molten glass, the step s9 of molding themolten material, and the step s10 of gradually cooling.

(The Step S5 of Preparing the Cullet 1)

The cullet is preferably carried out with the refractive indexmeasurement in advance, and in case the measured value of the refractiveindex is equal to the desired value, the cullet is used as the mixedcullet, and if the measured value of the refractive index does not matchthe desired value, the mixed cullet is formed by mixing the cullethaving the higher refractive index than the desired value and the cullethaving the lower value than the desired value.

The cullet of the present embodiment preferably satisfies the abovementioned equation (2), and preferably the cullet has high dissolved gasamount and excellent transparency effect. That is, the cullet ispreferably produced by adding the water vapor to the melting atmospherein the melting step (rough melting step). By using such cullet, forexample even in case the water vapor is not added to the meltingatmosphere of the re-melting step, the value of βOH of the glass and thecontent of the high refractive index component can be regulated tosatisfy the predetermined relationship, the dissolving amount of Pt orso can be reduced, and further excellent transparency can be exhibitedduring the refining step.

(The Step S6 of Heating and Melting the Cullet 1)

Next, the mixed cullet is introduced into the re-melting container, andthen it is heated and melted.

The container and the apparatus used for the re-melting can be selectedsuitably depending on the composition of the glass to be produced, andfor example the container or the apparatus made of noble metal (forexample, made of platinum or platinum alloy) or quartz may be used.Among these, the container and the apparatus made of platinum orplatinum alloy are preferable from the point comprising excellent heatresistance and excellent erosion resistance against the melting productmaterial during the melting.

As for the device of carrying out the re-melting step, the re-meltingdevice which carries out melting, refining and uniforming of the mixedcullet in one crucible, and also the re-melting device which comprisesplurality of tubs and carries out melting, refining and uniforming ineach tub can be used as well.

This device comprises the melting tub for melting the mixed cullet, therefining tub for refining the molten glass obtained by the melting, theprocessing tub to make the molten glass uniform after the refining andto adjust the viscosity to be suitable for molding, the connecting pipefor flowing the molten glass to the refining tub from the melting tub,the connecting pipe for flowing the molten glass to the processing tubfrom the refining tub, and the glass draining pipe for draining themolten glass inside the processing tub or so. In this device, onecontainer may be separated by placing a partition to form the meltingtub and refining tub.

As for the above mentioned tub, any known ones can be used.

Also, the melting temperature (the re-melting step) of the mixed culletduring the re-melting step is preferably within the range of 800 to1500° C. Note that, in order to increase the refining effect, it ispreferable to make this re-melting temperature lower than the refiningtemperature. The melting time during the re-melting step can be adjustedappropriately considering the capacity of the crucible, and the amountintroduced of the mixed cullet into the crucible. For example, themelting time during re-melting may be within the range of 2 to 20 hours.

The atmosphere during the melting is not particularly limited; howeverfrom the point of increasing βOH of the glass obtained at the end, thewater vapor is preferably added to the melting atmosphere.

By adding the water vapor to the melting atmosphere, the value of βOH ofthe optical glass obtained at the end and the content of the highrefractive index component can be regulated to satisfy the predeterminedrelationship, also the dissolving of Pt or so to the glass can beeffectively prevented during the production steps of the glass, and thedissolved gas can be supplied to the glass sufficient enough to improvethe transparency.

Particularly, for the both steps of the rough melting step and there-melting step, by adding the water vapor to the melting atmosphere,the value of βOH which has made high at the cullet state can bemaintained, and βOH can be made further higher thus the reducing effectof the coloring by the re-heat treatment can be increased. Also, byadding the water vapor to the atmosphere for the entire steps, oxygencan be effectively prevented from reacting with the melting containermade of noble metal such as platinum or so, and the dissolved amount ofPt into the glass can be reduced, thus the deterioration of thetransmittance can be prevented effectively. Further, the dissolved gassupplied to the cullet state can be maintained until right before therefining step, and the amount of the dissolved gas can be furtherincreased thus the effect of improving the transparency can be enhanced.

The method of adding the water vapor in melting atmosphere is notparticularly limited, but for example the method of introducing theconnecting pipe to the crucible from the opening part provided at themelting device, and depending on the needs, supplying the water vaporthrough this pipe to the space in the crucible may be mentioned.

The flow amount of the gas comprising the water vapor to be suppliedinto the space of the crucible is not particularly limited, and it canbe controlled based on the measured result of βOH of the glass which isproduced experimentally. For example, in case of supplying the watervapor in the melting container roughly sealed, the glass having thedesired βOH can be obtained by just supplying relatively small amount ofwater vapor. On the other hand, in case of melting the glass by placingthe crucible without the lid in the glass melting furnace, the volumeinside the glass melting furnace becomes larger compared to the volumeinside the crucible, thus in order to have desired βOH value, relativelylarge amount of the water vapor will be supplied into the glass meltingfurnace. Based on such experiment result, the supplying amount of thewater vapor; that is by feeding back the flow amount of the gas to thenext production, the glass having desired βOH value can be produced.Note that, hereinafter, the flow amount of the gas, the flow amount ofthe water vapor, the atmospheric adding flow amount, the supplyingamount of the water vapor are the value converted in 25° C. and 1atmospheric pressure.

The melting during the re-melting step is preferably carried out withthe bubbling in order to make the molten material uniform. The bubblingduring the re-melting is preferably continued after the mixed cullet hasbeen melted.

Note that, in case of not carrying out the bubbling, preferably themolten material is stirred and made uniform by other stirring methods.As for other stirring method, the known methods can be used and forexample by stirring with the stirring rod or so may be mentioned.

Also, the gas used for the bubbling is not necessarily limited, thus theknown gas can be used, and commercially available ones or the onegenerated can be used.

The gas used for the bubbling is preferably a gas including the watervapor, from the point that the value of βOH of the optical glassobtained at the end and the content of the high refractive indexcomponent can be regulated to satisfy the predetermined relationship,and also the dissolving of Pt or so to the glass can be effectivelyprevented. Further, from the point that the dissolved gas can besupplied to the glass sufficient enough to improve the transparency; thegas used for the bubbling is preferably the gas including the watervapor.

The flow amount of the gas comprising the water vapor which isintroduced into the molten material is not particularly limited, and itmay be regulated based on the measured result of βOH of the glass whichis produced experimentally. For example, when βOH of the glass producedexperimentally is measured and if the measured result is smaller thanthe desired value, the flow amount of the gas is increased; on the otherhand, if the measured result is larger than the desired βOH value, theflow amount of the gas is regulated to reduce the amount. As such, theflow amount of the gas can be regulated from the measured resultobtained by βOH of the glass produced experimentally. As such, based onthe measurement result of βOH of the glass produced experimentally, thesupplying amount of the water vapor, that is the flow amount of the gasis feed backed to the subsequent production, thereby the glass havingthe desired βOH can be produced.

The content of the water vapor in the gas comprising such water vapor ispreferably 10 vol % or more, more preferably 20 vol % or more, furtherpreferably 30 vol % or more, even more preferably 40 vol % or more, evenfurther preferably 50 vol %, furthermore preferably 60 vol %, evenfurthermore preferably 70 vol % or more, particularly preferably 80 vol% or more, and further particularly preferably 90 vol % or more. Thehigher the content of the water vapor is, the more preferable it is; andby setting within the above mentioned range, the value of βOH of theoptical glass obtained at the end and the content of the high refractiveindex component can be regulated to satisfy the predeterminedrelationship.

(The Refining Step S7 of the Molten Glass)

Once the cullet is completely melt and uniform molten glass is obtained,if the bubbling is carried out then the bubbling is terminated, then thetemperature of the molten glass is raised for the refining.

The refining temperature, that is, the temperature of the molten glassduring the refining step is preferably 900 to 1500° C. Note that, inorder to further enhance the refining effect, the refining temperatureis preferably higher than the temperature at the rough melting step andthe re-melting step. The refining time can be set so that the bubbleremaining in the glass becomes less than the predetermined amount andalso the coloring of the glass becomes less than the predeterminedvalue. It is effective to make the refining time longer for enhancingthe defoaming effect; however the molten glass will be maintained in thecrucible made of platinum or platinum alloy for long period of timeunder high temperature, thus the coloring of the glass may increase asplatinum dissolves into the molten glass, and the platinum contaminanttends to easily enter to the glass.

Therefore, the refining time is made short within the range that thesufficient bubble removal can be obtained, and to suppress the coloringof the glass. For example, the refining time may be within the range of1 to 10 hours.

(The Uniforming Step S8 of the Molten Glass)

After removing the bubble in the molten glass to the outside thereof bythe refining, the temperature of the molten glass is lowered, and themolten glass is uniformed by stirring.

The molten glass is uniformed by decreasing the temperature of themolten glass lower than the temperature of the refining temperature.During the uniforming step, the molten glass is made uniform bystirring. Not only the molten glass is made uniform during theuniforming step, it is the step to adjust the viscosity so that it issuitable for molding the molten glass. The uniforming time is adjustedaccordingly so that the striae is gone or is less by observing thepresence of the striae of the molded glass, and so that the viscosity ofthe molten glass is suitable for the molding.

(The Molding Step S9)

The molten glass being refined and uniformed is drained out from theglass draining pipe installed to the bottom part of the re-meltingcontainer, and then molds the glass by introducing into the mold.

The temperature of the glass draining pipe is within the temperaturerange that does not make the flowing molten glass devitrify, and it isadjusted and maintained so that the viscosity is suitable for themolding.

In the method of carrying out melting, refining and uniforming of theraw material in one crucible, a part of the glass draining pipe iscooled so that the glass inside is solidified, then the pipe is closedto carry out each step of melting, refining and uniforming. Then, partbeing cooled of the pipe is heated to melt the glass, and then the pipeis opened to drain the molten glass. The temperature regulation of theglass draining pipe may be done by known methods.

The molding of the molten glass may be carried out by known methods. Forexample, the molten glass is drained into the mold for molding.Alternatively, the molten glass bulk is separated from the molten glassand press-molded. Alternatively, the molten glass bulk is separated fromthe molten glass and it is molded while floating by applying the gaspressure.

(The Gradual Cooling Step S10)

Next, the molded glass is cooled gradually, then re-heating treatment iscarried out to remove the coloring and strain, and also the refractiveindex is adjusted finely thereby the optical glass of object isobtained.

The gradual cooling of the molded glass may be carried out by knownmethods. For example, the molded glass may be maintained at thetemperature near the glass transition temperature, and then graduallycooled by the predetermined temperature decreasing speed. Thepredetermined temperature decreasing speed differs depending on theglass composition, however for example it can be 0.1 to 100° C./hour.

The re-heating treatment is preferably carried out in the oxidizingatmosphere. Thereby, the coloring of the optical glass can be madesmall.

The glass obtained as such has extremely small content of noble metalsuch as Pt derived from the production apparatus such as the meltingcontainer or so. Therefore, the coloring of the glass due to theultraviolet ray so called solarization is little. As a result, theoptical element using the above mentioned glass has little change of thetransmittance over the time. Also, when fixing the optical element byusing an ultraviolet ray curable adhesive agent, it is possible toobtain the effect of which the transmittance does not decline even afterthe ultraviolet ray is irradiated to the optical element.

As for the gas used in the oxidizing atmosphere, it only needs to be agas with oxygen, and the oxygen concentration is for example about thesame of air or may be higher. As for such oxidizing atmosphere gas, forexample oxygen, air and the mixed gas thereof may be used. The heattreating temperature is preferably lower than the softening point of theglass, and higher than the temperature lower by 100° C. from the glasstransition temperature (Tg-100° C.).

Note that, when the coloring of the glass is reduced to a predeterminedlevel, the heat treating time can be shortened if the heat treatingtemperature is high. Also, the heat treating time can be shortened byincreasing the oxygen partial pressure in the oxidizing atmosphere. Theheat treating time as such changes depending on the heat treatingtemperature and the oxygen partial pressure in the oxidizing atmosphere;however it can be set so that the coloring of the glass is at thedesired level. The heat treating time is typically 0.1 hour to 100 hourspreferably.

Regarding the Glass Composition

Hereinafter, unless mentioned otherwise, the content of the glasscomponent, total content, the content of the additive will be expressedin mol % in terms of oxides.

The glass according to the present embodiment comprises at least oneoxide selected from TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ (hereinafter, it may besimply referred as “high refractive index component”) as the glasscomponent. Preferably, the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃included in the glass is 20% or more, more preferably 25% or more,further preferably 30% or more, and even further preferably 35% or more.If the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ exceeds 85%, then thedevitirification resistance tends to deteriorate, thus from the point tomaintain the devitrification resistance, the total content of TiO₂,Nb₂O₅, WO₃ and Bi₂O₃ is preferably 85% or less, more preferably 80% orless, and further preferably 75% or less.

From the point of increasing the content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃in the glass, in the production method of the present embodiment, theobtained glass is preferably P₂O₅ containing glass. In P₂O₅ containingglass, the moving speed of H⁺ during the heat treatment is fast, thusthe coloring can be reduced by the heat treatment of short time comparedto other composition type.

As for such glass, the glass wherein the content of P₂O₅ is larger thanthe content of SiO₂ and is larger than the content of B₂O₃; and theglass wherein the content of P₂O₅ is larger than the total content ofSiO₂ and B₂O₃ in terms of mol % expression, may be mentioned.

The present embodiment can be used for the glass composition comprisingthe known composition wherein the content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃are within the above mentioned range, in addition to the compositionshown in the examples.

Next, the preferable glass composition of the present embodiment will beexplained.

P₂O₅ is the glass network forming component, and it has the function tomaintain the thermal stability of the glass. If the content of P₂O₅ isless than 7%, the thermal stability tends to decline, thus preferablythe content of P₂O₅ is 7% or more. If the content of P₂O₅ is larger than40%, the refractive index declines. Therefore, the content of P₂O₅ ispreferably within 7 to 40%. The lower limit of the content of P₂O₅ is10%, more preferable lower limit is 12%, further preferable lower limitis 15%, and even more preferable lower limit is 18%. The preferableupper limit of the content of P₂O₅ is 35%, more preferable upper limitis 33%, further preferable upper limit is 30%, and even more preferableupper limit is 28%.

SiO₂ is difficult to be dissolved into the glass of P₂O₅ basedcomposition, and if it is introduced in a large amount, then undissolvedresidue will be generated hence the uniformity of the glass tends to bedeteriorated. Therefore, the content of SiO₂ is preferably less than thecontent (M) of P₂O₅. As for the relationship between the content of SiO₂and the above mentioned M (the content (%) of P₂O₅), the more preferablecontent of SiO₂ is 0% to 0.8×M [%], and further preferable range is 0%to 0.5×M [%], even preferable range is 0% to 0.3×M [%], and even morepreferable range is 0% to 0.15×M [%].

B₂O₃ function to improve the devitrification resistance by justcomprising a small amount. As for the relationship between the contentof B₂O₃ and the above mentioned M (the content (%) of P₂O₅), thepreferable content range of B₂O₃ is 0% or more and less than M [%], morepreferable range is 0% to 0.9×M [%], further preferable range is 0% to0.7×M [Vo], even preferable range is 0% to 0.6×M [%], even morepreferable range is 0% to 0.5×M [%], even further preferable range is 0%to 0.4×M [%], and still even more preferable range is 0% to 0.35×M [%].

TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ increases the refractive index, alsofunctions to increase the dispersion, and are components functions toimprove the chemical durability. However, if the contents of TiO₂,Nb₂O₅, WO₃ and Bi₂O₃ respectively become large, the devitrificationresistance tends to deteriorate.

From the point of maintaining the devitrification resistance, the upperlimit of the content of TiO₂ is 40%, more preferable upper limit is 35%,further preferable upper limit is 33%, and even more preferable upperlimit is 30%. From the point of obtaining introduction effect of TiO₂,the preferable lower limit of the content of TiO₂ is 1%, and morepreferable lower limit is 3%. The content of TiO₂ can be 0% as well.

From the point of maintaining the devitrification resistance, thepreferable upper limit of the content of Nb₂O₅ is 45%, more preferableupper limit is 40%, and more preferable upper limit is 35%. From thepoint of obtaining the introduction effect of Nb₂O₅, the preferablelower limit of the content of Nb₂O₅ is 5%, more preferable lower limitis 8%, and further preferable lower limit is 11%. The content of Nb₂O₅can be 0% as well.

The preferable range of the content of WO₃ is 0 to 30%. From the pointof obtaining the introduction effect of the above mentioned WO₃, thepreferable lower limit of the content of WO₃ is 1%, more preferablelower limit is 3%, and further preferable lower limit is 5%. On theother hand, from the point of obtaining the devitrification resistance,the preferable upper limit of the content of WO₃ is 27%, more preferableupper limit is 24%, further preferable upper limit is 20%, and even morepreferable upper limit is 18%. The content of WO₃ can be 0% as well.

The preferable range of the content of Bi₂O₃ is 0 to 35%. From the pointof obtaining the introduction effect of the above mentioned Bi₂O₃, thepreferable lower limit of the content of Bi₂O₃ is 1%, more preferablelower limit is 3%, and further preferable lower limit is 5%. On theother hand, from the point of obtaining the devitrification resistance,the preferable upper limit of the content of Bi₂O₃ is 30%, morepreferable upper limit is 28%, and further preferable upper limit is24%. The content of Bi₂O₃ can be 0% as well.

The divalent metal components such as BaO, SrO, CaO, MgO and ZnO or sofunctions to improve the melting property of the glass, and to reducethe coloring of the glass. Also, if it is an appropriate amount, itfunctions to improve the devitrification resistance. However, ifexcessive amount is comprised, the refractive index declines and thedevitrification resistance tends to deteriorate; thus the total contentof BaO, SrO, CaO, MgO and ZnO is preferably 0 to 40%, and morepreferably 0 to 32%. The preferable upper limit of the total content ofBaO, SrO, CaO, MgO and ZnO is 30%, more preferable upper limit is 27%,and further preferable upper limit is 25%. The preferable lower limit ofthe total content of BaO, SrO, CaO, MgO and ZnO is 0.1%, more preferableamount is 0.5%, and further preferable lower limit is 1%.

Among these divalent metal components, the content of BaO is preferablywithin the range of 0 to 40%, and more preferably within 0 to 32% sinceBaO is an effective component to maintain the high refractive index. Thepreferable upper limit of the content of BaO is 30%, more preferableupper limit is 27%, and further preferable upper limit is 25%. Thepreferable lower limit of the content of BaO is 0.1%, more preferablelower limit is 0.5% and further preferable lower limit is 1%. Thecontent of BaO can be 0% as well.

The alkali metal oxides such as Li₂O, Na₂O and K₂O or so functions toimprove the melting property of the glass, and reduces the coloring ofthe glass. Also, it functions to reduce the glass transition temperatureand the softening temperature, and functions to lower the heat treatingtemperature of the glass as well. However, if excessive amount iscomprised, the refractive index declines, and the devitrificationresistance tends to deteriorate, hence the total content of Li₂O, Na₂Oand K₂O is preferably 0 to 40%, more preferably 0 to 35%, furtherpreferably 0 to 32%, and even more preferably 0 to 30%. The content ofLi₂O, Na₂O and K₂O can be 0% as well. Particularly, in case of usingLi₂O as the alkali metal oxide, from the point of obtaining the highrefractive index glass, the content thereof in the produced glass ismore than 0% and less than 10%, more preferably more than 0% and 9% orless, and particularly preferably more than 0% and 8% or less.

Al₂O₃ function to improve the devitrification resistance if it is asmall amount, however if excessive amount is comprised, then therefractive index declines. Therefore, the preferable range of thecontent of Al₂O₃ is 0 to 12%, more preferable range is 0 to 7%, andfurther preferable range is 0 to 3%.

ZrO₂ function to enhance the refractive index, and if it is a smallamount, it functions to improve the devitrification resistance. However,excessive amount is comprised, the devitrification resistance and themelting property tends to deteriorate; thus the preferable range of thecontent of ZrO₂ is 0 to 16%, more preferable range is 0 to 12%, furtherpreferable range is 0 to 7%, and eve more preferable range is 0 to 3%.

GeO₂ function to maintain the devitrification resistance, and to enhancethe refractive index. Also, although GeO₂ function to enhance therefractive index, unlike TiO₂, Nb₂O₅, WO₃ and Bi₂O₃, it does notincrease the coloring of the glass. However, it is extremely expensivecomponent compared to other components, thus the lesser the content ofGeO₂ is, the better it is from the point of reducing the production costof the glass. Therefore, in order to widely spread the high refractiveindex glass product, it is demanded to provide the refractive glass withexcellent transmittance while reducing the content of GeO₂. According tothe present embodiment, by having 20% or more of the total content ofTiO₂, Nb₂O₅, WO₃ and Bi₂O₃, a high refractive index glass with excellenttransmittance can be provided without using large amount of GeO₂.

From the point of as such, the preferable range of the content of GeO₂.is 0 to 10%, more preferable range is 0 to 5%, further preferable rangeis 0 to 3%, even more preferable range is 0 to 2%, even furtherpreferable range is 0 to 1%, and even furthermore preferable range is 0to 0.5%; and GeO₂ may not be comprised. Note that, if the productioncost is not to be concerned, it can be suitably used in an effectiveamount.

TeO₂ maintain the devitrification resistance while functioning toimprove the refractive index. However, from the point of anenvironmental concern, the preferable range of the content of TeO₂ is 0to 10%, more preferable range is 0 to 5%, further preferable range is 0to 3%, even more preferable range is 0 to 2%, even further preferablerange is 0 to 1%, and even furthermore preferable range is 0 to 0.5%;and TeO₂ may not be comprised.

Sb₂O₃ has oxidizing effect, and it function to suppress the reduction ofTiO₂, Nb₂O₅, WO₃ and Bi₂O₃. However, Sb₂O₃ itself has an absorption inthe visible range, and facilitate the dissolving of the noble metal ionsto the molten glass by oxidizing the melting container made of noblemetal due to this oxidizing effect. Therefore, the preferable range ofthe content of Sb₂O₃ is 0 ppm or more and less than 1000 ppm. From theabove mentioned point of view, the upper limit of the content of Sb₂O₃is 900 ppm, 800 ppm, 700 ppm, 600 ppm, 500 ppm, 400 ppm, 300 ppm, 200ppm, 100 ppm in this order, and the smaller the value is the morepreferable it is. Sb₂O₃ may not be comprised.

If the component other than the above mentioned components is comprisedin a large amount, the devitrificaton resistance of the glassdeteriorates, and the liquidus temperature tends to increase. Therefore,the glass melting temperature must be increased, and an erosion of themelting container made of noble metal increases, thus the amount of thenoble metal dissolving into the glass increases. Also, the reduced colorof TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ increases as well.

From the point of suppressing the increase of such noble metal amountand to suppress the coloring of the glass, the total amount of P₂O₅,SiO₂, B₂O₃, TiO₂, Nb₂O₅, WO₃ and Bi₂O₃, MgO, CaO, SrO, BaO, ZnO, Li₂O,Na₂O, K₂O, Al₂O₃, ZrO₂, GeO₂, TeO₂ and Sb₂O₃ are preferably 90% or more,more preferably 92% or more, further preferably 95% or more, even morepreferably 96% or more, even further preferably 97% or more, still morepreferably 98% or more, and yet more preferably more than 99%. Notethat, the total content of the above mentioned may be 100%.

Ta₂O₅, Y₂O₃, La₂O₃, Gd₂O₃, Yb₂O₃, In₂O₃, Ga₂O₃, SnO₂, CeO₂ and F or somay be comprised in a small amount. The total content of Ta₂O₅, Y₂O₃,La₂O₃, Gd₂O₃, Yb₂O₃, In₂O₃, Ga₂O₃ and F is preferably 0 to 10%, morepreferably 0 to 7%, further preferably 0 to 5%, even more preferably 0to 3%, even further preferably 0 to 1%, and still more preferably 0 to0.5%.

F is a component which should not be included in a large amount from thepoint of increasing the volatility of the molten glass to obtain auniform glass, and to obtain the glass comprising the stable opticalcharacteristic. The preferable range of the content of F is 0 to 3%,more preferable range is 0 to 1%, further preferable range is 0 to 0.5%;and it is even more preferable to be substantially free of F.

From the point of reducing the environmental load, it is preferable tosubstantially be free of Pb, As, Cd, U, Th and Tl.

From the point of reducing the coloring of the glass, it is preferablysubstantially free of the additives and the components which haveabsorbance in the visible range such as Cu, Cr, Mn, Fe, Co, Ni, V, Mo,Nd, Eu, Er, Tb, Ho, Pr or so.

However, in the glass production method of the present embodiment, theinevitable impurities are not excluded.

Note that, as the glass raw material, depending on the glass components,the known glass raw material can be used such as oxides, phosphoricacid, phosphates (polyphosphate, metaphosphate, pyrophosphate or so),boric acid, boric anhydride, carbonates, nitrates, sulfates, hydroxidesor so.

The Production of the Optical Element

In order to make the optical element by using the optical glass of theabove, the known methods can be used. For example, the molten glass ismolded to produce the glass material for press-molding. Next, this glassmaterial is re-heated, and press molded to produce the optical elementblank. Further, the optical element is produced by processing by thestep including the polishing of the optical element blank.

Alternatively, the molten glass is molded to produce the glass materialfor press-molding, and this glass material is heated and the precisionpress-molding is carried out to produce the optical element.

In the above mentioned each step, the molten glass is molded to producethe glass mold product, and the glass mold product is processed toproduce the glass material for press-molding.

Alternatively, the molten glass is molded to produce the glass moldproduct, and this glass mold product is processed to produce the opticalelement.

To the optical function face of the produced optical element,anti-reflection film, total reflection film or so may be coateddepending on the purpose of the use.

As for the optical element, various lenses such as spherical lenses,macro lenses, lens array or so, prism, diffraction gratings or so may bementioned as examples.

Hereinabove, the embodiment of the present invention has been described,however the present invention is not to be limited thereto; and variousembodiments can be carried out within the scope of the present inventionwhich does not exceed the gist of the present invention.

For example, in the present embodiment, the production of the glass bythe rough melting-re-melting method has been described; however theknown methods can be employed such as the method of obtaining the glassby heating, melting and molding the batch raw material (the batch directmelting method) or so.

Also, in the present embodiment, as the method to increase βOH of theglass, the method of adding the water vapor to the melting atmospherehas been mainly described; however the method of bubbling the gasincluding the water vapor to the molten material, or the method of usingthe compound including the water as the glass raw material or so may bementioned. These methods may be combined for used.

Note that, in case of the method of increasing the water content in themolten glass by using the compound (for example orthophosphoric acid orboric acid or so) including the water as the glass raw material, thewater transpires from the molten glass, hence it is difficult tosufficiently increase βOH of the glass by this method alone. Therefore,the method of using the compound including the water as the glass rawmaterial, it is preferable to use together with above mentioned othermethods.

Also, the glass according to the present embodiment is suitable as thematerial for the optical element, thus it is preferably an amorphousglass. As the method of producing the optical element made of glass, forexample the method of heating, softening and molding the glass materialmay be mentioned. The crystalline glass wherein the crystalline phase isdispersed in the vitreosity is not suitable for the molding method ofthe above mentioned. Also, the crystalline phase of the crystallineglass scatters the light, and it may lower the performance as theoptical element. As for the amorphous glass, there is no such problem.

Also, in the embodiment of the present invention, the optical glass isused as the example, however as long as it is a glass product of whichthe coloring due to the reducing component causes problem, it can besuitably used for the production of various glass product not only forthe optical elements. As for such glass product, for example, opticalwindow material, solar battery glass, cover glass or so may bementioned.

Also, the present embodiment mentions the method of melting the rawmaterial using mainly the crucible as one example for the production ofthe optical glass, however as for the melting container, the tube madeof quartz or so and with opened both ends or so may be used.

Specifically, in the glass melting furnace, the tube made of quartz isfixed by being inclined. At the bottom part of the glass meltingfurnace, the opening part is provided to the position corresponding tothe lower part of the opening end of the lower position side of thetube. The raw material (the batch raw material or the cullet) isintroduced into the tube from the opening end of the higher positionside of the tube, then melt (or dissolve) inside the tube, thereby formsthe molten material. The molten material slowly flows inside of thetube, and flows out from the opening side of the lower position side ofthe tube.

For example in case of using the above mentioned tubes or so, during therough melting step, the draining product passes through the opening partof the bottom of the furnace, and is dropped in to the water of thewater tank placed in advance at the lower side of the opening part ofthe bottom part of the glass melting furnace, thereby forms the cullet.

In the above mentioned method, the raw material is melted using the tubemade of the quartz, however instead of the tube, the crucible made ofquarts or so may be used as well. First, the raw material is placedinside the crucible made of quartz, and heated and melted to form themolten material, then the molten material may be casted in the water, ordrained out on to the heat resistance board which has been cooledthereby the cullet may be produced.

Next, other embodiment of glass with high water content as the modifiedexample of the above mentioned main embodiment will be shown in below.

The Embodiment According to the First Modified Example

The present embodiment is about the same as the above mentionedembodiment except that the equation to determine the lower limit of βOHof the glass differs as shown in below, and the overlapping descriptionwill be omitted in below.

In the present embodiment, the main objects are to reduce the dissolvingof the noble metal into the molten glass and to improve thetransparency. The glass according to such embodiment has the refractiveindex nd of 1.75 or more, and the value of pox shown in the belowequation (1) satisfies the relation shown in the below equation (6).

βOH=−[ln(B/A)]/t  (1)

βOH≧181.39×nd ⁻³−325.75×nd ⁻²+194.85×nd ⁻¹−38.1  (6)

“t” is as mentioned in above, the thickness (mm) of the glass used forthe measurement of the external transmittance. The unit of βOH is rnm⁻¹.In the equation (6), “nd” is the refractive index of said glass at thewavelength 587.56 nm (d-line of the yellow helium). The refractive indexnd of the glass according to the present embodiment is 1.75 or more.Also, the lower limit of the refractive index nd is preferably 1.80,more preferably 1.85, and further preferably 1.90. Also, the upper limitof the refractive index nd is not particularly limited as long as theglass is obtained, and for example it can be 2.5 or so. By using theoptical element made of glass with high refractive index andconstituting the optical system, the optical system becomes more compactand show higher performance. From such point of view, the higher therefractive index nd is, the more preferable it is. However, as therefractive index becomes higher, the devitrification resistance tends todecline. Therefore, from the point of maintaining the devitrificationresistance, the upper limit of the refractive index nd is preferably2.4, and more preferably 2.3.

Also, in the glass according to the present embodiment, the value of βOHshown in the below equation (1) satisfies the relation shown in thebelow equation (7), and more preferably it satisfies the relation shownin below equation (8).

βOH≧181.39×nd ⁻³−325.75×nd ⁻²+194.85×nd ⁻¹−38.05  (7)

βOH≧181.39×nd ⁻³−325.75×nd ⁻²+194.85×nd ⁻¹−38.00  (8)

Also, the upper limit of βOH differs depending on the type of the glassand the production condition or so, and it is not particularly limitedas long as it can be adjusted. If βOH is increased, the amount of thevolatile product from the molten glass tends to increase, hence from thepoint of suppressing the volatilization from the molten glass, βOH is 10mm⁻¹ or less, preferably 8 mm⁻¹ or less, more preferably 6 mm⁻¹ or less,even preferably 5 mm⁻¹ or less, even further preferably 4 mm⁻¹ or less,even more preferably 3 mm⁻¹ or less, and still even further preferably 2mm⁻¹ or less.

In the glass according to the present embodiment, the value of βOHsatisfies the relation shown in the above mentioned equation (6). Thatis, the glass according to the present embodiment has higherconcentration of water in the glass compared to the glass produced bythe usual production method. This is because the glass according to thepresent embodiment has been actively taking in the water to the glass bythe procedure to increase the water content in the molten glass duringthe production steps thereof. Here, as the procedure to increase thewater content in the molten glass, for example the treatment to add thewater vapor to the melting atmosphere, or the treatment of bubbling thegas including the water vapor in the molten material or so may bementioned.

Also, for the melting of the glass as mentioned in the above, ingeneral, the melting container produced by the noble metals such asplatinum, gold, rhodium, iridium or so, or the alloy of these noblemetals are used; however, these noble metal materials dissolve into themolten material when melting the glass, and causes the solarization orthe coloring of the glass.

The glass according to the present embodiment has little dissolvingamount of the noble metal even in case the noble metal such a platinumor so is used as the melting container or the melting apparatus. Thatis, the glass according to the present embodiment has significantlylittle content of the noble metal even in case noble metals areincluded.

From the point of reducing the coloring of the glass caused by the noblemetal ion, improving the transmittance, reducing the solarization, andreducing the noble metal contaminant or so, the content of the noblemetal in the obtained glass is 4 ppm or less. The lower the upper limitof the content of the noble metal is, the more preferable it is, and itis further preferable to have lower upper limit in the order of; 3 ppm,2.7 ppm, 2.5 ppm, 2.2 ppm, 2.0 ppm, 1.8 ppm, 1.6 ppm, 1.4 ppm, 1.2 ppm,1.1 ppm, 1.0 ppm, 0.9 ppm. The lower limit of the content of the noblemetal is not particularly limited; however 0.001 ppm or so will beincluded inevitably.

As the noble metal, a metal simple substances such as Pt, Au, Rh, Ir orso, and alloy such as Pt alloy, Au alloy, Rh alloy, Ir alloy or so maybe mentioned. As for the melting container material or the meltingapparatus material, Pt or Pt alloy is preferable as it has heatresistance and corrosion resistance among the noble metals. Therefore,for the glass produced using the melting container and melting apparatusmade of Pt or Pt alloy, the content of Pt comprised in the glass ispreferably 4 ppm or less. As for more preferable upper limit of thecontent of Pt, it is the same as the further preferable content of thenoble metal included in the glass. Also, the lower limit of the contentof Pt is not particularly limited; however 0.001 ppm or so will beincluded inevitably.

Hereinafter, the example wherein the melting container is platinum (Pt)will be used for explanation, however same applies to even when themelting container made of metal material other than noble metal such asplatinum.

The glass according to the present embodiment has been carried out withthe procedure to increase the water content in the molten glass duringthe production steps thereof. When such treatment is carried out duringthe production steps of the glass, the oxygen partial pressure in themelting atmosphere is reduced, and the oxidation of the noble metalmaterial such as platinum or so which is the material of the meltingcontainer (the crucible or so) is prevented.

As a result, platinum dioxide or platinum ion (Pt⁴⁺), produced due tothe reaction between oxygen and the platinum material or so under themelting atmosphere can be effectively prevented, from dissolving intothe molten material (glass); and the dissolved amount of Pt in theobtained glass can be reduced even more.

Usually, the noble metal ion dissolved in the molten glass absorbs thevisible light, thus the coloring of the glass increases which is aproblem. However, the glass according to the present embodiment issufficiently reduced with the content of Pt as mentioned in the above,thus the coloring derived from Pt ion is little and has excellenttransmittance.

Also, the glass according to the present embodiment has excellenttransparency. Therefore, the time needed for the refining step can beshortened, thus the production cost can be reduced significantly.

Generally, the transparency of the glass depends on the amount of thedissolved gas in the molten glass. Such dissolved gas amount is largelyinfluenced by the composition of the glass (particularly of the type ofthe raw material), and the melting time and the melting number. However,if the dissolved gas can be supplemented during the melting step, theproblem of the transparency can be solved.

The glass according to the present embodiment has actively taken in thewater to the glass by the procedure to increase the water content in themolten glass during the production steps thereof. As a result, thedissolved gas as the water vapor in the molten glass can besupplemented, and the transparency of the glass can be improved.

The glass according to such embodiment, as mentioned in above, has beencarried out with the procedure to increase the water content in themolten glass during the production steps. The glass according to thepresent embodiment which has gone through such treatment takes in thewater to the molten glass during the melting step thereof, thus it hashigher concentration of water and higher βOH in the glass compared tothe glass with the same composition produced by the usual productionmethod.

Thereby, the present inventors have speculated that by carrying out thetreatment to increase βOH to the obtained glass, the dissolving of Ptcan be reduced and the transparency can be improved.

The method to increase βOH of the glass is not particularly limited;however preferably the procedure to increase the water content in themolten glass during the melting step may be mentioned. Here, as theprocedure to increase the water content in the molten glass, for examplethe treatment to add the water vapor in the melting atmosphere, or thetreatment to bubble the gas including the water vapor in the moltenmaterial or so may be mentioned.

Usually, according to these methods, the water can be introduced in theglass, and βOH can be increased, however the increasing rate thereofdiffers depending on the glass. As a result of keen examination by thepresent inventors, it was found that the easiness to take the water intothe glass depends on the refractive index nd of the glass. That is, thelarger the refractive index nd of the glass is, the more difficult thewater is to be taken in.

Therefore, for example, the glass with relatively low refractive indexnd easily takes in the water, and by carrying out the procedure toincrease βOH as mentioned in the above, βOH of the glass can be improvedsignificantly. However, the glass with relatively high refractive indexnd scarcely take in the water, thus even if the treatment is carried outunder the same condition, it is difficult to increase the value of βOHto the same level as the glass having the high refractive index, thusβOH of the obtained glass becomes low.

As such, the easiness to take the water into the glass differs dependingon the refractive index rid of the glass. Therefore, the presentinvention has defined the above equation (6) and determined the lowerlimit of βOH depending on the glass composition.

Here, in the above mentioned equation (6), “nd” refers to the refractiveindex of the glass.

As discussed in above, there is a glass which easily takes the water independing on the refractive index nd of the glass, and those which arenot. As a result of keen examinations, the present inventors have foundthat as the higher the refractive index nd of the glass is, the watertends to be difficult to be taken in, thereby the above mentionedequation (6) is determined.

Such equation (6) differentiates whether the glass has been carried outwith the treatment to increase βOH during the production steps thereof.That is, during the glass production steps thereof, the glass which isnot carried out with the treatment to increase βOH (the glass producedby the conventional production method) does not satisfy the abovementioned equation (6).

As the components to increase the refractive index nd of the glass, forexample, the high refractive index components such as Ti, Nb, W, Bi orso may be mentioned, however in the glass comprising these highrefractive index in a large amount, usually these high refractive indexcomponents are reduced during the melting of the glass, and the light atthe short wavelength side of the visible light range is absorbed, hencethe coloring in the obtained glass may increase in some cases.

The coloring of such glass (hereinafter, it may be referred as thereduced color) is reduced by carrying out the re-heating treatment underthe oxidizing atmosphere. This is thought because the high refractiveindex component at the reduced condition is carried out with there-heating treatment under the oxidizing atmosphere, and due to theoxidation, the visible light absorption of each ion is weakened.

Particularly, in order to reduce the coloring in short period of time,it is necessary to make the speed of oxidizing the reduced highrefractive index component at the heat treating faster, and to do so, itis necessary to have ions which can oxidize reduced high refractiveindex components by moving inside the glass in a speedy manner and givethe electric charge. As for such ion, H⁺ is thought to suitable.

Here, the glass according to the present embodiment satisfies the abovementioned equation (6). That is, sufficient water is introduced in theglass, and H⁺ derived from the water is present in a large amount. As aresult, due to the re-heating treatment, H⁺ moves inside the glass in aspeedy manner and give the electric charge, thus each ion of the reducedhigh refractive index component can be efficiently oxidized. Thereby, inthe glass according to the present embodiment, the coloring can besignificantly reduced by the heat treatment of the short period of time,and the glass after the re-heating treatment has excellenttransmittance.

Note that, since the infrared light transmit through even a dark coloredglass, thus βOH can be evaluated regardless of the coloring of theglass. Also, usually, since the re-heating treatment is carried outunder a lower temperature than the softening point of the glass, thevalue of βOH of the glass does not substantially change before and afterthereof, thus it may be measured any time before and after there-heating treatment. Therefore, βOH of the glass can be measured by anyof the transparent glass after the re-heating treatment (the treatmentto reduce the coloring) and the glass with dark color which has not gonethrough the re-heating treatment.

The glass of the present embodiment is not particularly limited as longas the above mentioned equation (6) is satisfied, and the treatment todecrease the reduced color may be carried out, or it may not be carriedout with this treatment.

Also, the glass according to the present embodiment can be suitably usedfor the optical glass.

Usually, for the optical glass, an excellent transmittance and thetransparency are demanded. In this regard, the optical glass of thepresent embodiment has the content of Pt which is significantly reduced,thus the coloring derived from Pt is extremely little, and has excellenttransmittance, while the dissolved gas amount in the molten glass isincreased and has excellent transmittance, further the glass of uniformand with little bubble can be obtained in short period of time.

Further, the optical glass according to the present embodiment canreduce the coloring efficiently by the re-heating treatment even in caseof comprising large amount of high refractive index component.

Note that, the glass according to the present embodiment can be producedby the same method as the glass according to main embodiment of theabove.

The Embodiment According to the Second Modified Example

The optical glass of the first embodiment according to the secondmodified example has the refractive index nd of 1.9 or more and lessthan 1.97, and it is an oxide glass including at least one oxidesselected from the group consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as theglass component, wherein the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃is within the range of 30 mol % to 60 mol %, and the value of βOH shownin the below equation (1) satisfies 0.1 mm⁻¹ or more.

βOH=−ln(B/A)/t  (1)

Also, the optical glass of the second embodiment according to the secondmodified example has the refractive index of 1.97 or more, and it is anoxide glass including at least one oxides selected from the groupconsisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as the glass component, whereinthe total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the range of40 mol % to 80 mol %, and the value of βOH shown in the below equation(1) satisfies 0.1 mm⁻¹ or more.

βOH=−ln(B/A)/t  (1)

In the following description, “the optical glass material” is the glassproduced via the molding step which molds the molten glass in themelting container to a predetermined shape, and refers to the glasshaving dark coloring of before the heat treatment. Also, “the opticalglass” refers to the glass of which the optical glass material havingdark color is heat treated. That is, “the optical glass” is the glasswherein the coloring is reduced than “the optical glass material” bycarrying out the heat treatment. Also, “the optical glass material” and“the optical glass”, and “the glass material for press-molding”, “theoptical glass” and “other optical glass product” produced by “theoptical glass material” or “the optical glass” is glass having theamorphous form, and it is not crystalline glass.

In the optical glass of the first and the second embodiment, even thoughthe high refractive index component selected from the group consistingof TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is comprised by a large amount within theabove mentioned range, the coloring is little. For the reason sucheffect can be obtained is speculated as below by the present inventors.

First, when melting the glass having high refractive index including thehigh refractive component such as TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ using themelting container of the noble metal such as platinum, the metal ion issuppressed from dissolving into the molten glass by placing the moltenglass to the reduced side when carrying out the melting. However, if themolten glass is reduced too much, as previously mentioned, the meltingcontainer turns into alloy. Also, even if the molten glass is notreduced too much, but if the coloring degree of the glass increases bythe high refractive index component being reduced, the degree of thereduction of the coloring will be only small even after the heattreatment is carried out to this glass in the subsequent step.

In order to overcome such problem, the optical glass material may bemade by forming a condition wherein the metal material constituting themelting container does not dissolve into the molten glass, and bycarrying out the heat treatment to the obtained optical glass material,the coloring can be reduced significantly.

The present inventors speculate regarding the phenomena of coloringreduction of the glass due to the heat treatment as follows. First, thecoloring of the optical glass can be reduced by carrying out the heattreatment of the optical glass material under the oxidizing atmosphere,however each ion of Ti, Nb, W, Bi or so which are in the reduced stateare oxidized, and the visible light absorbance of each ion becomesweaker. If the speed of oxidizing Ti, Nb, W and Bi are slow, theimprovement of the coloring will be small even if the optical glassmaterial is heat treated. In order to significantly reduce the coloringof the optical glass material, the oxidizing speed of Ti, Nb, W and Biduring the heat treatment can be made larger. If there is an ion whicheasily moves in the glass, and as long as this ion does not directlyinfluence the coloring, such ion may make speedy movement in the glassand may give the electric charge, thereby it would be possible to reducethe coloring in short period of time caused by the reduction of Ti, Nb,W, Bi which has been reduced. As for such ion, H⁺ is thought to besuitable; however, in order to make H⁺ further easier to move, OH⁻ isintroduced in the glass structure to allow the hopping of H⁺ from OH⁻,thereby it is thought that the oxidation speed during the heat treatmentcan be increased.

In order to introduce H⁺ and OH⁻ in the optical glass material, H₂O isto be introduced in the optical glass material. Here, the water contentof the optical glass material can be quantified indirectly by measuringthe infrared absorbance intensity by Off for the optical glass withlittle coloring and improved transmittance.

Therefore, in the optical glass material having the refractive index of1.9 or more and less than 1.97, which is an oxide glass including atleast one oxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃and Bi₂O₃ as the glass component and the total content of TiO₂, Nb₂O₅,WO₃ and Bi₂O₃ is within the range of 30 mol % to 60 mol %; or in theoptical glass material having the refractive index of 1.97 or more,which is an oxide glass including at least one oxide selected from thegroup consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as the glass componentand the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the rangeof 40 mol % to 80 mol %; by having the value of βOH, which is theindicator of the content of OH⁻ in the optical glass, of 0.1 mm⁻¹ ormore in the optical glass material, the coloring can be made little.

Further, when producing the optical glass material including lots ofwater such that the value βOH is 0.1 mm⁻¹ or more, the procedure such asthe addition of the water vapor to the melting atmosphere, or thebubbling of the water vapor to the molten material is carried out. Theseprocedures reduces the oxygen partial pressure in the meltingatmosphere, hence the oxidation of the metal material (including thealloy material) constituting the melting container used for the meltingof the molten glass is suppressed. As a result, the dissolving amount ofthe metal material to the molten glass is reduced, and the increase ofthe coloring caused by the dissolving of the metal material (includingthe alloy material) can be suppressed. Note that, the value of βOH canbe measured for the optical glass material with dark coloring as same asthe optical glass, since the infrared light can transmit through theoptical glass material.

Next, in regards with the optical glass of the first and the secondembodiment, the reason why the lower limit of the value of βOH has beenset to 0.1 mm⁻¹ will be described.

FIG. 3 is a graph showing the change of the external transmittance(T450) at the wavelength of 450 nm when the light enter parallel to thethickness direction of the No. 1 glass having the thickness of 5 mm withrespect to βOH value when βOH value of the No. 1 glass is changed fromthe composition of the Table 1. Note that, the value of the externaltransmittance (T450) shown in FIG. 3 is the value of after the heattreatment of the No. 1 glass for 1 hour at 600° C. in the air, and thevalue of βOH is also the value after the heat treatment. Also, No. 1glass has the refractive index nd of 1.9 or more, and the total contentof TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the range of 30 mol % to 60 mol%. That is, No. 1 glass is the same as the optical glass of the firstembodiment for the refractive index nd and the glass composition.

TABLE 1 Glass component (mol %) No. 1 No. 2 P₂O₅ 25.43 23.43 B₂O₃ 4.074.45 SiO₂ 1.18 1.29 TiO₂ 26.6 15.5 Nb₂O₅ 25.04 24.74 WO₃ 0 0 Bi₂O₃ 0 0Na₂O 10.28 4.99 K₂O 6.01 2.46 BaO 1.39 21.24 ZnO 0 1.9 Total 100 100TiO₂ + N₂ O₅ + WO₃ + Bi₂O₃ 51.64 40.24 Refractive index nd 1.9546 1.922Abbe number vd 17.9 20.9 Glass transition temperature — — Tg (° C.)Liquidus temperature LT (° C.) 1100 1080

Also, FIG. 4 is a graph showing the change of the external transmittance(T450) at the wavelength of 450 nm when the light enter parallel to thethickness direction of the No. 3 glass having the thickness of 5 mm withrespect to βOH value when βOH value of the No. 3 glass is changed fromthe composition of the Table 2. Note that, the value of the externaltransmittance (T450) shown in FIG. 4 is the value of after the heattreatment of the No. 3 glass for 4.5 hours at 570° C. in the air, andthe value of βOH is also the value after the heat treatment. Also, No. 3glass has the refractive index nd of 1.97 or more, and the total contentof TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the range of 40 mol % to 80 mol%. That is, No. 3 glass is the same as the optical glass of the secondembodiment for the refractive index nd and the glass composition,

TABLE 2 Glass component (mol %) No. 3 No. 4 P₂O₅ 22.579 25.5 B₂O₃ 2.8262.0 SiO₂ 1.613 0 TiO₂ 18.148 7.0 Nb₂O₅ 16.535 18.0 WO₃ 14.515 8.0 Bi₂O₃20.966 20.0 Li₂O 0 6.0 Na₂O 0 10.5 K₂O 0 2.0 BaO 2.818 1.0 ZnO 0 0 Total100 100 TiO₂ + N₂ O₅ + WO₃ + Bi₂O₃ 70.164 53.0 Refractive index nd2.10639 2.003 Abbe number vd 17.01 19.1 Glass transition temperature562.5 486 Tg (° C.) Liquidus temperature LT (° C.) 970 920

Further, the five values of βOH shown in FIG. 3 and FIG. 4 are the valueset by regulating the amount of the water vapor introduced into theglass melting atmosphere when melting No. 1 glass and No. 3 glass. Asobvious from FIG. 3 and FIG. 4, as the values of βOH increases theexternal transmittance (T450) increases as well. Also, according to thetrends of the change of the external transmittance (T450) against thevalues of βOH shown in FIG. 3, it is understood that when the value ofβOH is 0.1 mm⁻¹ or more, the external transmittance (T450) exceeds 30%for sure. Also, according to the trend of the change of the externaltransmittance (T450) against the values of βOH shown in FIG. 4, it isunderstood that when the value of βOH is 0.1 mm⁻¹ or more, the externaltransmittance (T450) exceeds 10% for sure.

As such, for both No. 1 glass and No. 3 glass, by extending the heattreatment time, the external transmittance (T450) further increases, andthe suitable transmittance characteristic as the material of the opticalelement can be obtained.

When fixing the optical element to the lens barrel or so, the processingproperty can be significantly improved by using the adhesive agent ofultraviolet ray curable type. For the optical element using the opticalglass having high refractive index of the refractive index nd of 1.9 ormore or the refractive index nd of 1.97 or more, the light of the shortwavelength side at the visible light range is cut by the optical glass,thus it was difficult to cure by irradiating the adhesive agent curinglight across the optical element. However, by improving thetransmittance of the short wavelength side at the visible light range,it is possible to adhere which uses the adhesive agent of theultraviolet ray curable type.

Therefore, considering the light transmitting property of the opticalelement constituting the optical system and the convenience of adhesivehandling when using the ultraviolet ray curable adhesive agent, in caseof the optical glass according to the first embodiment, the lower limitof βOH is preferable in the increasing order of, 0.2 mm⁻¹, 0.3 mm⁻¹, 0.4mm⁻¹, 0.5 mm⁻¹, 0.6 mm⁻¹, 0.7 mm⁻¹, 0.8 mm⁻¹. Also, in case of theoptical glass according to the second embodiment, the lower limit of βOHis preferable in the increasing order of, 0.15 mm⁻¹, 0.2 mm⁻¹, 0.25mm⁻¹, 0.3 mm⁻¹, 0.35 mm⁻¹, 0.4 mm⁻¹, 0.45 mm⁻¹, 0.5 mm⁻¹, 0.55 mm⁻¹, 0.6mm⁻¹, 0.65 mm⁻¹, 0.7 mm⁻¹, 0.75 mm⁻¹, 0.8 mm⁻¹, 0.85 mm⁻¹, 0.9 mm⁻¹. Bymaking the value of βOH large as such, the external transmittance (T450)increases, and the coloring of the optical glass can be easily madelittle.

Note that, the optical glass of the first and the second embodiment ispreferably phosphate based glass. The phosphate based glass tends totake the water in easier than borate based glass, thus the coloring ofthe optical glass can be reduced further easier.

In this case, the optical glass of the first embodiment preferablyincludes 15 mol % to 35 mol % of P₂O₅ as the glass component. By makingthe content of P₂O₅ to 15 mol % or more, the water content in theoptical glass can be increased, and the value of βOH can be made furtherlarger easily. On the other hand, by making the content of P₂O₅ to 35mol % or less, the high refractive index becomes easy to maintain. Notethat, the preferable lower limit of the content of P₂O₅ is 17 mol %, andthe preferable upper limit is 33 mol %.

Also, the optical glass of the second embodiment preferably includes 10mol % to 35 mol % of P₂O₅ as the glass component. By making the contentof P₂O₅ to 10 mol % or more, the water content in the optical glass canbe increased, and the value of βOH can be made further larger easily. Onthe other hand, by making the content of P₂O₅ to 35 mol % or less, thehigh refractive index becomes easy to maintain. Note that, thepreferable lower limit of the content of P₂O₅ is 12 mol %, and thepreferable upper limit is 33 mol %.

Not that, the coloring degree of the optical glass can be quantified byλτ80 which is the indicator to show the coloring degree. λτ80 refers tothe wavelength (nm) wherein the internal transmittance (internaltransmittance τ) is 80% which is calculated first by measuring theinternal transmittance at the range of the wavelength 280 to 700 nm whenthe light enter into the optical glass parallel to the thicknessdirection thereof, then assuming that the thickness of the optical glassbased on the internal transmittance measured is 10 mm. Here, theinternal transmittance τ is the transmittance excluding the surfacereflection loss at the incident side and the emitting side; and is avalue obtained by measuring the transmittance T1, T2 including thesurface reflection loss of each sample using two samples with differentthickness, that is by carrying out the measurement of the externaltransmittance T1, T2 within the wavelength range of 280 nm to 1550 nm,and calculated based on the following equation (9) using these measuredvalue.

log τ=−(log T1−log T2)×10/Δd  (9)

Here, in the equation (9), T1 is the transmittance (%) including thesurface reflection loss measured in the wavelength range of 280 nm to1550 nm when the light enters parallel to the thickness direction offirst sample, wherein the thickness of the first sample is d1 (mm). T2is the transmittance (%) including the surface reflection loss measuredin the wavelength range of 280 nm to 1550 nm when the light entersparallel to the thickness direction of second sample, wherein thethickness of the second sample is d2 (mm) made of same glass as thefirst sample. Note that, λτ80 is calculated using the result of thetransmittance measurement at the wavelength of 280 to 700 nm, thus themeasurement of the transmittance T1 and T2 may be carried out within thewavelength range of 280 to 700 nm. Also, Δd is the difference d2−d1 (mm)between the thickness d1 and the thickness d2; and the thickness d1 andthe thickness d2 satisfies the relation of d1<d2.

λτ80 increases as the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃increases. In case the total content of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ inmol % expression is X, when the optical glass is produced by heattreating the optical glass material without increasing the water contentin the optical glass material prior to the heat treatment in theoxidizing atmosphere, X and λτ80 satisfies the below equation (10).

Therefore, it is difficult to drastically improve λτ80.

λτ80>aX+b  (10)

Note that, in the equation (10), “a” is the constant (1.8359 nm/mol %),and “b” is the constant (351.06 nm).

On the other hand, in the production method of the optical glass of thefirst embodiment, λτ80 can be reduced so that it satisfies the belowequation (11).

λτ80<aX+b  (11)

Note that, in the equation (11), “a” and “b” are the same as theequation (10).

Note that, the optical glass of the first and the second embodimentpreferably satisfies the below equation (12), and further preferablysatisfies the equation (13).

λτ80<aX+c  (12)

λτ80<aX+d  (13)

Here, in the equation (12), “a” and “b” are the same as the equation(10). In equation (13), “a” and “b” are the same as the equation (10) aswell. In equation (12), “c” is the constant (348.06 nm). Also, in theequation (13), “d” is the constant (345.06 nm).

According to the optical glass of the first and the second embodiment,when λτ80 or more and within the wavelength range of 700 nm or less, theinternal transmittance converted to the thickness of 10 mm is 80% ormore; and preferably even when λτ80 or more and within the wavelengthrange of 1550 nm or less, preferably the internal transmittanceconverted to the thickness of 10 mm is 80% or more.

Note that, conventionally, antimony oxide having an oxidation effect hasbeen added since it suppresses the reduction of the high refractiveindex component during the glass melting. However, when producing theoptical glass of the first and the second embodiment, the coloring canbe made less without using the oxidation effect of the antimony oxide.Further, by adding the antimony oxide, the metal material constitutingthe melting container is ionized by being oxidized, and dissolves intothe optical glass material, and it may cause the coloring of the opticalglass obtained at the end. Therefore, in the optical glass of the firstand the second embodiment, it is preferable to have the content of theantimony oxide of less than 1000 ppm and more preferably less than 700ppm in terms of Sb₂O₃. Note that, the upper limit of the content ofantimony oxide is preferable in the order of 600 ppm, 500 ppm, 400 ppm,300 ppm, 200 ppm, 100 ppm; and it is further preferable to be less thanthese values. Further, the optical glass of the first and the secondembodiment may be free of antimony oxide.

The optical glass of the first and the second embodiment preferably hasthe composition including P₂O₅ and at least one oxide selected from thegroup consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃; and in addition to this,it is further preferable to have the composition including alkali metaloxides, alkali earth metal oxides, ZnO, B₂O₃, SiO₂ or so as arbitrarycomponents. Even for the optical glass comprising such composition, thecontent of P₂O₅ and the total content of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃are within said preferable range.

Also, in the optical glass of the first and the second embodiment,alkali metal oxides such as Li₂O or so may be included. Here, in case ofusing Li₂O, from the point of obtaining high refractive glass, thecontent there of is preferably more than 0 mol % and less than 10 mol %,more preferably more than 0 mol % and 9 mol % or less, and furtherpreferably more than 0 mol % and 8 mol % or less. Also, in the opticalglass of the first and the second embodiment, GeO₂ and/or Ga₂O₃ may beincluded. Note that, since these oxides are expensive, Ga₂O₃ may not beincluded in the optical glass, however in case it is included, and it ispreferable to reduce the content thereof as less as possible. Here, incase GeO₂ is included in the optical glass, the content thereof ispreferably more than 0 mol % and 5 mol % or less, more preferably morethan 0 mol % and 2 mol % or less, and further preferably more than 0 mol% and 1 mol % or less. Also, in case Ga₂O₃ is included in the opticalglass, the content thereof is preferably more than 0 mol % and 0.5 mol %or less, more preferably more than 0 mol % and 0.2 mol % or less, andfurther preferably more than 0 mol % and 0.1 mol % or less. The opticalglass of the first and the second embodiment may not include Li₂O, maynot include GeO₂ and may not include Ga₂O₃.

In the optical glass of the first and the second embodiment, consideringthe environmental influence, it is preferable to be free of Pb, As, Cd,U, Th as the glass component. From the point of preventing the increaseof the coloring, it is preferable to be free of the component whichabsorbs the visible light such as Cr, Ni, Eu, Er, Tb, Fe, Cu, Nd or so.Te may be included in the within the range which does not interfere theobject of the present invention; however from the point of theenvironmental influence, it is preferably not included as the glasscomponent. Note that, in the present specification, by referring “notinclude”, it does not exclude the amount which is inevitably included asthe impurities.

Next, the production method of the optical glass of the first and thesecond embodiment will be described.

In the production method of the optical glass of the first embodiment,the optical glass material which is the oxide glass comprising at leastone oxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃ andBi₂O₃ as a glass component, in which the total content of said TiO₂,Nb₂O₅, WO₃ and Bi₂O₃ is 30 mol % to 60 mol % is produced by at leastgoing through the heating and melting step of the glass raw material inthe melting container to obtained the molten glass, and the molding stepof molding the molten glass in the melting container into apredetermined shape. Next, this optical glass material is carried outwith the heat treating in the oxidizing atmosphere; thereby the opticalglass of the first embodiment is obtained.

In the production method of the optical glass of the second embodiment,the optical glass material which is the oxide glass comprising at leastone oxide selected from the group consisting of TiO₂, Nb₂O₅, WO₃ andBi₂O₃ as a glass component, in which the total content of said TiO₂,Nb₂O₅, WO₃ and Bi₂O₃ is 40 mol % to 80 mol % is produced by at leastgoing through the heating and melting step of the glass material in themelting container to obtained the molten glass, and the molding step ofmolding the molten glass in the melting container into a predeterminedshape. Next, this optical glass material is carried out with the heattreating in the oxidizing atmosphere; thereby the optical glass of thesecond embodiment is obtained.

Here, during the heating and melting step, by regulating the watercontent included in the molten glass in the melting container, the valueof βOH of the optical glass may be controlled to 0.1 mm⁻¹ or more.

In the production method of the optical glass of the first and thesecond embodiment, as for the regulating method of the glass material,the heating method and the melting method of the glass material, and themolding method of the molten glass, the known methods can be usedappropriately. Also, from the point of obtaining the uniform opticalglass, the melting container is preferably constituted by the metalmaterial. Here, as for the metal material constituting the meltingcontainer, noble metals such as platinum or gold, and noble metal alloyssuch as platinum alloy and gold alloy or so are preferable since it hasexcellent erosion resistance and heat resistance.

Here, as the regulating method of the water content included in themolten glass, it is preferable to use any one of the first water contentregulating method supplying the water vapor to the atmosphere of whichis melting the molten glass, the second water content regulating methodsupplying the water vapor by bubbling in the molten glass, and the thirdwater content regulating method combining the first water contentregulating method and the second water content regulating method. Notethat, by referring as regulating the water content included in themolten glass in the melting container, it mainly refers to the procedureto increase the water content included in the molten glass as mentionedin the first to third water content regulating methods.

Note that, as the regulating method of the water content included in themolten glass in the melting container, the method of using the compoundincluding the water as the glass raw material, for example the method ofincreasing the water content in the molten glass by using the glass rawmaterial with orthophosphoric acid or boric acid. However, in thismethod, the water evaporates during the process of melting the glass rawmaterial, thus it is difficult to secure the sufficient water content inthe optical glass material and the optical glass. Further, in the methodwherein the compound is mixed to make the raw material, and carrying outthe rough melting of this raw material to form the cullet, thenre-melting the cullet followed by re-melting in the melting container,the water which was originally included in the raw material is lost, andwhen the re-melting is carried out in the melting container, the watercontent is significantly reduced.

Therefore, in case of using the compound including the water such asorthophosphoric acid or boric acid as the glass raw material, it ispreferable to increase the water vapor partial pressure in the meltingatmosphere by suppressing the evaporation of the water from the moltenglass. Alternatively, in case the compound including the water as theglass raw material, the melting container is made air-tight, thereby thewater vapor may be made not to evaporate out of the melting containerduring the heating and melting step. Such procedure is also included forthe regulation of the water content included in the molten glass in themelting container.

Note that, the heating and the melting step includes the melting step ofheating the glass raw material and melting the mold glass, the refiningstep facilitating the bubble removal of the molten glass, and theuniforming step of uniforming and stirring the molten glass of after therefining by decreasing the temperature so that the viscosity is suitablefor the molding. In case of using the cullet as the glass raw material,the cullet forming step of carrying out the rough melting of the glassraw material mixed with the aforementioned compound so called the batchraw material to form the cullet is carried out before the melting step.

Even for the method of producing the cullet, or even for the method ofmelting the batch raw material in the direct melting step, from thepoint of suppressing the excessive reduction of Ti, Nb, W and Bi, and incase the melting container is constituted from the metal material tosuppress the ionization of the metal material thereof, and therebysecuring the water content in the optical glass material and the opticalglass, it is preferable to maintain the heating temperature of the glassduring the heating and melting temperature to 1400° C. or less, andfurther preferably maintain to 1300° C. or less. Further, from the pointof improving the transparency and the obtaining the optical glass withlittle coloring, it is preferable to set so that the heating temperatureof the glass during the heating and melting step is at the highest atthe refining step, that is it is preferable to melt the glass at thetemperature below the refining temperature.

Also, if the time from the start to the end of the heating-melting stepis too long, it promotes the reduction of the high refractive indexcomponent, and in case the melting container is made of metal material,the ionization of the metal material thereof is facilitated, thus thewater content in the optical glass tends to decline. Therefore, it ispreferable that the time from the start to the end of theheating-melting step is within 100 hours. Note that, the time from thestart to the end of the heating-melting step may be adjustedappropriately depending on the size of the capacity of the meltingcontainer. By carrying out the heat treatment of the optical glass beingmelted and molded as such in the oxidizing atmosphere, the coloring ofthe optical glass can be made small.

As the oxidizing atmosphere gas, the air, the gas added with the oxygenin the air, and oxygen or so may be used. Also, the heat treatingtemperature and the heat treating time is preferably set so that λτ80satisfy the equation (11), and more preferably set so that λτ80satisfies the equation (12), and further preferably set so that λτ80satisfies the equation (13).

The glass material for press-molding of the present embodiment and theoptical element of the present embodiment include the optical glass ofthe first and the second embodiment, and in general only consisted fromthe optical glass of the first and the second embodiment.

The glass material for press-molding is the glass material for obtainingthe press-molding product, specifically the optical blank or the opticalelement by heating and melting the optical glass and press molding. Asthe production method of the press molding glass material, for examplethe method of separating the flowing molten glass flow to form themolten glass bulk and molding into the press molding glass materialduring the process of cooling this molten glass bulk; and the method ofmolding the glass block by introducing the molten glass and forming thepress molding glass material by processing the glass block.

As for the example of the optical element, various lenses such as thespherical lenses, non-spherical lenses, and prism or so may bementioned. The optical element of the present embodiment can be producedby going through the subsequent processing step wherein the opticalglass of the present embodiment is subsequently processed. As thesubsequent processing, various known subsequent processing such as heattreating, molding, polishing or so can be carried out appropriately, anddepending on the needs, two or more of the subsequent processingtreatments can be combined. As for the method of producing the opticalelement by the subsequent processing, the method of producing theoptical element blank by heating and softening the optical glass (or thepress molding glass material) then press molding, and processing theoptical element blank; and the method of obtaining the optical elementby producing the optical element blank by press molding the molten glassthen processing the optical element or so may be mentioned.

Note that, when producing the press molding glass material and theoptical element, it may be produced by using the optical glass materialused for the production of the optical glass first and the secondembodiment, and carrying out various processing such as the molding andthe polishing or so, then carrying out the heat treating for reducingthe coloring.

EXAMPLE

Hereinbelow, the present invention will be described based on theeaxamples, however the present invention is not to be limited thereto.

Example 1 The Preparation of the Batch Raw Material

First, for producing the optical glass comprising the desiredcharacteristics, phosphoric acid, barium metaphosphate, titanium oxide,niobium oxide, tungsten oxide, bismuth oxide, boric acid, bariumcarbonate, sodium carbonate, potassium carbonate and silicon oxide wereprepared as the glass raw material. Next, the above mentioned rawmaterials were accordingly selected, scaled and thoroughly mixed so thatthe glass composition of the optical glass which will be obtained at theend satisfies the oxide composition I to VIII shown in Table 3, therebythe batch raw materials I to VIII were produced.

TABLE 3 Glass Oxide composition (mol %) component I II III IV V VI VIIVIII P₂O₅ 19.8 21.6 23.6 24.6 25.7 30.4 22.6 21.5 TiO₂ 11.3 17.0 11.819.3 26.7 21.2 18.2 21.2 Nb₂O₅ 21.9 15.0 29.3 28.2 26.3 20.6 16.5 19.3WO₃ — — — — — 9.0 14.5 9.4 Bi₂O₃ — — — — — 10.1 20.9 24.5 B₂O₃ 15.1 10.7 6.2 5.0 3.8 1.9 2.8 2.4 BaO — 17.0 22.1 12.1 1.5 2.1 2.8 1.7 Na₂O 27.213.6  7.0 5.0 10.0 3.0 — — K₂O  4.7  5.1 — 5.8 6.0 1.7 — — SiO₂ — — — —— — 1.6 —

[The Production of the Cullet and the Mixed Cullet (the Rough MeltingStep)]

The batch raw materials I to VIII being mixed was made as the glass rawmaterial of each optical glass. This glass raw material was introducedinto the crucible made of quartz, and melted at 900 to 1350° C. in theair atmosphere, thereby obtained the molten material. The moltenmaterial obtained as such was dropped into the water to obtain thecullet.

The cullet which was taken out of the water was dried, and a part of thecullet was sampled for the refractive index measurement, and melted byplacing in the crucible made of platinum, then refined the obtainedglass molten liquid and was made uniform. Then, it was introduced in themold for molding, and maintained at the temperature near the glasstransition temperature, then cooled at the temperature decreasing speedof 30° C./hour. The refractive index nd of the refractive indexmeasurement sample obtained as such was measured by the refractive indexmeasurement method in accordance with Japan Optical Glass IndustrySociety Standard.

Next, depending on the measured refractive index nd, the cullet wasmixed so that it satisfies the desired refractive index, therebyobtained the mixed cullet for the production of the optical glass.

[The Production of the Optical Glass (Re-Melting Step)]

Next, the mixed cullet was introduced into the crucible made of platinum(melting container), and within the range of 800 to 1350° C., the mixedcullet in the crucible made of platinum was heated and melted to formthe molten glass (melting step).

Then, the temperature of the crucible was increased to the refiningtemperature (900 to 1450° C.) for refining (refining step). Then, thetemperature of the crucible was cooled to uniforming temperature, thenstirred using the stirring apparatus thereby it was uniformed(uniforming step).

Note that, the volume of the melting furnace (the volume of the spaceinside the furnace of flame resistant which houses the crucible) and theplacement time of the molten material in the melting furnace (the timefrom the introduction of the cullet to the platinum crucible containeruntil the molten glass drains out from the melting container) are shownin Table 4.

Also, for carrying out the melting step, refining step, and uniformingstep, the procedure to increase the water content in the molten glasswas carried out depending on the needs.

Specifically, the pipe made of platinum was inserted from the outside ofthe furnace into the crucible made of platinum placed inside thefurnace, and the water vapor (H₂O 100 vol %) was supplied to the spaceinside the crucible made of platinum via this pipe made of platinum. Assuch, the addition of the water vapor to the melting atmosphere wascarried out by adding the water vapor to the air. The flow amount of thesupplied water vapor is shown in Table 4.

Also, if necessary, the water vapor (H₂O 100 vol %) was bubbled into themolten material from the tube provided at the lower part of thecrucible. As such, the bubbling of the water vapor to into the moltenmaterial was carried out by bubbling the water vapor to the moltenmaterial in the air atmosphere or to the molten material in the meltingatmosphere added with the water vapor to the air. The flow amount of thewater vapor supplied is shown in Table 4.

Note that, the flow amount of the water vapor shown in Table 4 is thevalue converted to the flow amount at the usual temperature and usualpressure, and the unit is littler/min.

Also, in case the water vapor is not supplied, the lid made of platinumwas not used, and while the melting container was kept opened, themelting step to the uniforming step via the refining step were allcarried out under the air atmosphere.

TABLE 4 The procedure to increase the water content Atmospheric Place-adding flow bubbling flow Sample Oxide Volume ment amount amount No.composition litter time litter/min litter/ min 11 I 40 4.5 — — 12 93 8.6— — 13 40 4.5  15 — 14 40 4.5  40 — 15 40 4.5 320 — 16 40 4.5 320 4 21II 40 4.8 — — 22 93 9.1 — — 23 121 9.8 — — 24 40 4.8  40 — 25 40 4.8 320— 26 40 4.8 350 — 31 III 40 5.5 — — 32 93 9.7 — — 33 40 5.5 250 — 34 405.5 300 — 35 40 5.5 320 — 41 IV 40 5.2 — — 42 93 9.7 — — 43 40 5.2  10 —44 40 5.2 250 — 45 40 5.2 300 — 46 40 5.2 320 — 51 V 40 7.8 — — 52 939.1 — — 53 40 4.8  15 — 54 40 4.8  40 — 55 40 4.8 320 — 56 40 4.8 320 461 VI 40 6.5 — — 62 93 9.1 — — 63 40 6.5  15 — 64 40 6.5  40 — 65 40 6.5300 — 66 40 6.5 320 — 71 VII 40 7.3 — — 72 40 7.3  2 — 73 6 5.0  34 — 81VIII 40 7.3 — — 82 40 7.3  2 — 83 6 5.0  12 — 84 6 5.0  34 —

The molten glass which has been uniformed as such was drained out fromthe glass draining pipe made of platinum installed to the bottom part ofthe crucible (the draining step) in the air atmosphere, and byintroducing into the mold placed at the lower side of the draining pipe,a long glass block (the width of 150 mm×the thickness of 10 mm) wasmolded (the molding step).

Then, the above mentioned glass block was increased with the temperatureat the speed of +100° C./hour, and maintained for 1.5 to 8 hours at thetemperature near each glass transition temperature, then cooled at thespeed of −10° C./hour (the annealing step), thereby the optical glasssample removed with the strain was obtained.

[The Evaluation of the Optical Glass]

Various physical properties of the obtained optical glass sample (sample11 to sample 84) were measured and evaluated as in below.

[1] The Glass Composition

The appropriate amount of the optical glass sample was taken, andtreated with acid and alkaline, then using the inductively coupledplasma mass spectrometry method (ICP-MS method) and the ionchromatography method, the content of each component was quantitativelymeasured to confirm that it matches with the oxide composition I toVIII.

[2] The Refractive Index Nd, Abbe Number Vd and the Glass TransitionTemperature Tg

The molten glass which has gone through the uniforming step whenproducing the optical glass sample was molded by introducing into themold, and maintained at the temperature near the glass transitiontemperature, then cooled at the temperature decreasing speed of 10°C./hour to produce the measuring sample. For the obtained measuringsample, the refractive index nd, ng, nF, nc were measured in accordancewith Japan Optical Glass Industry Society Standard. Further, by thesemeasured value of the refractive indexes, Abbe number vd was calculated.

Next, optical glass sample was processed, and the measurement sample ofcolumn shape (the diameter of 5 mm and the height of 20 mm) wasproduced. For the obtained measurement sample, by using thethermomechanical analysis apparatus (TMA) and under the condition of thetemperature rising speed of +10° C./min, the glass transitiontemperature Tg was measured.

Note that, these characteristic values were derived from the glasscompositions, thus for the optical glass sample using the same batch rawmaterial as the glass raw material were confirmed to have substantiallythe same values. The results are shown in Table 5.

TABLE 5 Oxide composition I II III IV V VI VII VIII Refractive index nd1.81 1.87 1.92 1.93 1.95 2.02 2.11 2.16 Abbe number vd 22.5 21.8 20.919.2 18.0 17.8 17.0 16.2 Glass transition 541 604 666 652 637 601 5.61558 temperature Tg (° C.)

[3] βOH

The optical glass sample was processed, and then the plate shaped glasssample having the thickness of 1 mm being optically polished so that theboth faces are flat and parallel to each other was prepared. To thepolished face of this plate shaped glass sample, the light was enteredin vertical direction, then the external transmittance A at thewavelength of 2500 nm, and the external transmittance B at thewavelength of 2900 nm were measured using the spectrophotometer, and βOHwas calculated from the below equation (1).

βOH=−[ln(B/A)]/t  (1)

In the above mentioned equation (1), In is a natural logarithm, and thethickness t corresponds to the space between the two planar faces of theabove mentioned. Also, the external transmittance includes thereflection loss at the glass sample surface, and it is the ratio (thetransmitted light intensity/the incident light intensity) of theintensity of the transmitted light against the intensity of the incidentlight entering to the glass sample. Also, the higher the value of βOHis, the more water is included in the glass. The results are shown inTable 8 and FIG. 2.

FIG. 2 is the graph plotting βOH of each optical glass sample for eachglass composition. In FIG. 2, the bold line shows the border lineseparating the example and the comparative example defined by the belowequation (2).

βOH≧0.4891×ln(1/HR)+2.48  (2)

Note that, the value which separates the example and the comparativeexample of each composition (the lower limit value of βOH which canexpect to exhibit the effect of the present invention) can be calculatedby the above mentioned equation (2). That is, by the composition ratioof the above shown in Table 3, HR is calculated (the total content (mol%) of each component TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ in the glass component),and introduces the above equation (2). The value calculated based oneach oxide composition is shown in Table 8. The unit of βOH is mm⁻¹.

[4] T450 (H)

The optical glass sample was increased with the temperature at the speedof +100° C./hour in the air atmosphere, and maintained for 100 hours atthe predetermined maintaining temperature, then the temperature wasdecreased at the speed of −30° C./hour, thereby the heat treatment wascarried out. Note that, the maintaining temperature differs depending onthe composition, thus it was set to temperature shown in Table 6depending on the oxide compositions of each optical glass sample.

TABLE 6 Oxide composition I II III IV V VI VII VIII Maintaining 530 600650 630 630 570 550 530 temperature (° C.)

The optical glass sample carried out with the heat treatment wasprocessed, and then the plate shaped glass sample having the thicknessof 10 mm being optically polished so that the both faces are flat andparallel to each other was prepared. For the plate shaped glass sampleobtained as such, the external transmittance T450(H) at 450 nm wasobtained using the spectrophotometer. The larger the value of T450(H)is, the more excellent the transmittance is, and it means that thecoloring of the glass is reduced. The results are shown in Table 8.

[5] Pt Content

An appropriate amount of the optical glass sample was taken, and thiswas carried out with alkali fusion to separate Pt, then Pt amount in theglass was quantified by TCP-MS method. The results are shown in Table 8.

[6] The Coloring Degree λ80 and λ70

First, the optical glass sample was heat treated under the samecondition as T450(H).

The optical glass sample carried out with the heat treatment wasprocessed, and then the plate shaped glass sample having the thicknessof 10 mm±0.1 mm being optically polished so that the both faces are flatand parallel to each other was prepared. To the polished face of thisplate shaped glass sample, the light entered in a vertical direction,and the spectral transmittance including the surface reflection losswithin the range of the wavelength of 280 nm to 700 nm was measuredusing the spectrophotometer; then the wavelength wherein the spectraltransmittance (the external transmittance) becomes 80% and 70% weredetermined as the coloring degree λ80 and λ70 respectively. The smallerthe value of each of λ80 and λ70 are, the lesser the coloring of theglass is. The results are shown in Table 8. Note that, for the sampleevaluated for λ80, the underline is shown in the result of Table 8.

[7] T450 (L)

0.5 to 0.7 cc of the molten glass which has gone through the uniformingstep during the production of the optical glass sample was taken, andintroduced into the concave part of the mold for a float molding (themold having a structure wherein the concave part receiving the moltenglass is formed of porous materials, and the gas was spurted out fromthe surface of the concave part via the porous material), then the gasspurts out from the concave part and upward gas pressure was applied tothe molten glass on the concave part, thereby the glass bulk whilefloating was produced.

Then, the above mentioned glass bulk was increased with the temperatureat the speed of +100° C./hour, and maintained at the predeterminedmaintaining temperature for predetermined maintaining time, then thetemperature was lowered at the speed of −30° C./hour, thereby obtainedthe spherical optical glass sample of after the heat treatment. Notethat, the maintaining temperature and the maintaining time differsdepending on the composition, hence the temperature and the time wereset as shown in Table 7 depending on the oxides composition of eachoptical glass sample.

TABLE 7 Oxide composition I II III IV V VI VII VIII Maintaining 500 550650 630 600 600 550 500 temperature (° C.) Maintaining 2 4 4 4 1 5 5 6time (h)

The obtained spherical optical glass sample was processed, and then theplate shaped glass sample having the thickness of 5 mm being opticallypolished so that the both faces are flat and parallel to each other wasprepared. For the plate shaped glass sample obtained as such, theexternal transmittance T450(L) at 450 nm was obtained using thespectrophotometer. The larger the value of T450(L) is, the moreexcellent the transmittance is, and it means that the coloring of theglass is reduced even after the heat treatment of short period of time.

[8] The Defoaming

40 cc of the molten glass (glass molten liquid) before the refining stepduring the production of the optical glass sample was taken, and wasrefined for certain period of time by separate platinum crucible, thenthe glass molten liquid was cooled down in the platinum crucible therebyit was solidified. During this step, the coloring was reduced so thatthe number of bubbles included in the glass can be counted. Next, thesolidified glass was taken out from the platinum crucible.

The samples for the measurement obtained as such was observed bymagnifying (100×) the inside of the glass using the optical microscope(the magnification of 20 to 100×), then the number of bubbles includedin the glass was counted. The same observations were carried out to eachsample for the measurement sample with different refining times, and therefining time of the measurement sample wherein the number of thebubbles remaining in the glass to be 100/kg were evaluated as the timefor removing the bubbles. The shorter the defoaming is, the moreexcellent the refining property is. The results are shown in Table 8.

TABLE 8 Oxide Bubble composition β-OH/ T450(H) Pt λ80/λ70 T450(L)removal Sample No. equation (2) mm % ppm nm % min 11 I 0.46 79.4 2.40460 75.7 92 12 βOH ≧ 0.77 0.58 79.4 2.00 457 80.0 84 13 0.80 80.0 1.40446 81.4 78 14 1.15 81.2 0.97 435 82.5 72 15 1.60 81.7 0.54 427 82.1 6116 1.97 81.8 0.26 423 84.3 55 21 II 0.39 77.4 2.80 417 76.9 88 22 βOH ≧0.78 0.48 78.6 2.40 412 77.9 79 23 0.65 79.7 1.80 407 81.9 75 24 1.1080.0 1.20 406 82.8 71 25 1.50 80.5 0.83 405 84.5 66 26 1.55 81.4 0.45403 82.5 64 31 III 0.54 75.7 2.00 427 69.0 83 32 βOH ≧ 0.66 0.61 76.51.90 425 75.1 81 33 0.83 77.1 1.70 415 78.4 78 34 1.14 78.2 0.61 41182.2 77 35 1.18 78.8 0.49 409 81.4 75 41 IV 0.34 72.2 3.50 438 61.1 9742 βOH ≧ 0.59 0.43 74.5 2.80 425 66.6 92 43 0.66 76.8 1.90 417 72.3 8744 0.94 77.3 1.30 416 78.2 84 45 1.13 77.9 0.87 414 79.9 83 46 1.34 78.50.62 413 80.2 76 51 V 0.25 68.5 2.80 457 34.5 83 52 βOH ≧ 0.54 0.51 73.12.30 437 58.1 75 53 0.69 74.6 1.50 429 67.8 74 54 1.02 75.7 1.10 42671.2 72 55 1.31 77.1 0.64 421 75.0 71 56 1.53 77.3 0.36 420 79.3 70 61VI 0.35 67.3 3.10 461 28.0 89 62 βOH ≧ 0.47 0.46 70.0 2.70 450 49.8 8763 0.66 72.5 1.60 441 62.7 84 64 0.89 73.9 1.20 437 70.1 82 65 1.15 74.80.88 435 73.3 75 66 1.29 75.6 0.62 433 73.4 71 71 VII 0.35 56.1 3.00 48525.6 85 72 βOH ≧ 0.40 0.52 58.0 1.76 473 57.4 74 73 0.72 63.8 0.67 46064.1 66 81 VIII 0.28 51.3 3.30 521 17.5 103 82 βOH ≧ 0.37 0.39 54.8 1.90502 25.2 94 83 0.52 57.0 1.50 494 44.9 88 84 0.65 61.2 0.88 480 56.4 83

As shown in Table 8 and FIG. 2, when the glass of the present inventionwherein βOH of the optical glass sample satisfies the above mentionedequation (2), the improvement effect of the transmittance due to theheat treatment is significant, and the dissolved amount of Pt derivedfrom the melting container is also significantly reduced, thus excellenttransmittance was confirmed (samples 13 to 16, samples 24 to 26, samples33 to 35, samples 43 to 46, samples 53 to 56, samples 63 to 66, sample72, sample 73, and samples 82 to 84).

On the other hand, when βOH of the glass does not satisfy the abovementioned equation (2), then it corresponds to the comparative exampleof the present invention, and has small improving effect of thetransmittance due to the heat treatment and the dissolved amount of Ptderived from the melting container is large, thus the transmittance waslow (sample 11, sample 12, samples 21 to 23, sample 31, sample 32,sample 41, sample 42, sample 51, sample 52, sample 61, sample 62, sample71, and sample 81).

Also, in case of the glass of the present invention, sufficientimproving effect of the transmittance by the heat treatment can beobtained compared to the case of the glass which corresponds to thecomparative example of the present invention, and also it was confirmedthat the time needed for the bubble removal was short. That is, in caseof the glass of the present invention, the time needed for the refiningstep and the heat treating step can be shortened significantly, andduring the production of the optical glass, the production can bereduced and the productivity can be improved.

Example 2

The optical glass samples (samples 51a to 56a) were produced under thesame condition as the samples 51 to 56 of the example 1 expect thatantimony oxide (Sb₂O₃) were added to the batch raw material V as theglass material. The added amounts of antimony oxide are shown in Table9. Note that, the unit is ppm with respect to 100 wt % of batch rawmaterial.

[The Evaluation of the Optical Glass]

Various physical properties of the obtained optical glass samples (thesamples 51a to 56a) were measured and evaluated under the same conditionas the example 1.

As a result, the refractive index nd, Abbe number vd and the glasstransition temperature Tg were substantially the same as the valuesshown in oxide composition of the example 1. The results are shown inTable 9.

TABLE 9 Procedure to increase the water content Atmospheric bubblingPlacement adding flow flow λ80/ Sample Sb2O3 Volume time amount amountβ-OH/ T450(H) Pt λ70 No. ppm litter hour litter/min litter/min mm % ppmnm 51a 3000 40 7.8 — — 0.25 64.3 3.0 467 52a 3000 93 9.1 — — 0.51 64.72.5 447 53a 155 40 4.8 15 — 0.69 74.4 1.6 430 54a 155 40 4.8 40 — 1.0275.4 1.1 427 55a 150 40 4.8 320 — 1.31 76.9 0.66 422 56a 100 40 4.8 3204 1.53 77.2 0.37 421

As shown in Table 9, depending on the presence of the antimony oxide inthe glass, it was confirmed that the value of βOH of the glasssubstantially has no change (samples 51 to 56, and samples 51a to 56a).

Also, even in case of using the batch raw material added with antimonyoxide, the optical glass sample produced according to the presentinvention was confirmed to have excellent transmittance even after theheat treatment, and also it was confirmed that the amount of Ptdissolved in the glass was reduced (samples 53a to 56a).

Example 3

The optical glass samples (the glass blocks) produced in the examplesland 2 were divided, and depending on the needs, further processing wascarried out, thereby the glass material for press-molding correspondingto each optical glass was obtained.

The glass material for press-molding obtained as such was heated andsoftened in the air then press-molded, thereby the optical element blankclose to the lens shape was produced.

Next, the optical element blank obtained was annealed in the air, thenthe processing such as grinding and polishing were carried out, therebythe glass made optical element made of glass such as lens and prism orso corresponding to each sample of the examples 1 and 2 were produced.

Note that, the temperature decreasing speed during the annealing was setso that the refractive index of the optical element becomes the desiredvalue.

Also, for the press-molding method of the glass, the annealing method,the grinding method and the polishing method or so of the lens blank,the known methods were used.

The optical element produced by using the optical glass sample (samples13 to 16, samples 24 to 26, samples 33 to 35, samples 43 to 46, samples53 to 56, samples 63 to 66, samples 72, samples 73, samples 82 to 84,samples 53a to 56a) was confirmed with the significant coloringreduction by carrying out the heat treatment in the oxidizing atmospheresuch as air or so in between the molding of the molten glass and theprocessing of the optical element blank.

On the other hand, the optical element produced by using the opticalglass samples (sample 11, sample 12, samples 21 to 23, sample 31, sample32, sample 41, sample 42, sample 51, sample 52, sample 61, sample 62,sample 71, sample 81, sample 51b and sample 52b) produced by theproduction method corresponding to the comparative example of thepresent invention had the coloring remaining, and the coloring reductioneffect was confirmed to be low even after going through the heattreatment in the oxidizing atmosphere of air or so in between themolding of the molten glass and the processing of the optical elementblank.

Examples according to the first modified example

Next, FIG. 5 shows the graph plotting βOH of the optical glass sampleproduced in the example 1 of the first example in terms of eachrefractive index nd of the glass from the point of the first modifiedexample.

In FIG. 5, the bold line shows the border line separating the exampleand the comparative example based on the below equation (6).

βOH≧181.39×nd ⁻³−325.75×nd ⁻²+194.85×nd ⁻¹−38.1  (6)

Here, “nd” in the equation (6) shows the refractive index of said glass.

Examples according to the second modified example

Next, the examples according to the second modified example will beshown. Note that, for the second modified example, it is not to belimited to the below examples. Also, herein below, the number of theexamples will be renewed.

Examples 1 to 6

The batch raw material was carried out with the rough melting to producethe cullet, and the cullet was placed in the crucible made of platinumand heated, melted and molded, then each optical glass having thecomposition shown in No. 1 to No. 4 of Table 1 and Table 2 were producedby the below described order.

First, phosphates, orthophosphoric acid, oxides, carbonates, nitrates,sulfates were scaled and thoroughly mixed to prepare the raw material(the batch raw material). Next, this batch raw material was placed intothe container made of quartz, and the optical glass of No. 1 and No. 2were heated within the range of liquidus temperature 800 to 1400° C.,and the optical glass of No. 3 and No. 4 were heated at the range ofliquidus temperature LT to 1300° C., thereby molten glass was made, andthe cullet raw material was produced by dropping this molten glass intothe water.

Next, the cullet raw material was dried, then the cullet raw materialwas re-mixed, and introduced into the crucible made of platinum (themelting container) then the lid made of platinum was placed on. Whileunder this condition, the cullet raw materials in the crucible made ofplatinum were heated in the range of liquidus temperature LT to 1300° C.for the glass composition of the cullet raw material of the opticalglass of No. 1 and No. 2, and in the range of liquidus temperature LT to1250° C. for the glass composition of the cullet raw material of theoptical glass of No. 3 and No. 4; then the cullet war material weremelted and molten glass was obtained (melting step).

Further, for the optical glass of No. 1 and No. 2, after refining themolten glass by raising the temperature within the range of the liquidustemperature LT to 1400° C. (the refining step), the temperature wasdecreased within the range of the liquidus temperature LT to 1300° C.;and for optical glass of No. 3 and No. 4, after refining the moltenglass by raising the temperature within the range of the liquidustemperature LT to 1300° C. (the refining step), the temperature wasdecreased within the range of the liquidus temperature LT to 1250° C.Then, these were uniformed by stirring (the uniforming step), and themolten glass being refined and uniformed were introduced into the moldby draining out from the glass draining pipe. Thereby, the glass blockwas molded.

Note that, when carrying out the melting step, the refining step, theuniforming step, the pipe made of platinum was inserted into thecrucible made of platinum from the opening part provided at the lid madeof platinum, and depending on the needs, the water vapor was able to besupplied to the space in the crucible made of platinum via this pipemade of the platinum. The flow amount of the water vapor per unit timesupplied into the crucible made of platinum is shown in Table 10. Notethat, the flow amount of the water vapor shown in Table 10 is the valueconverted in the flow amount at usual temperature, and the unit islitter/min. Also, in case the water vapor is not supplied into thecrucible, the crucible made of platinum is covered by the lid made ofplatinum and without the opening part; and the water was suppressed fromevaporating from the cullet material and the molten glass during themelting by sealing the crucible made of platinum between the meltingstep to the uniforming step via the refining step.

Next, each glass block made of the optical glass of No. 1 and theoptical glass of No. 2 was increased with the temperature to 600° C.from 25° C. in air taking 2 hours, then annealed at 600° C. (the heattreatment), then carried out with the procedure to reduce the coloringof the glass block (the optical glass material). Then, the glass blockwas cooled to the usual temperature at the temperature decreasing speedof −30° C./hour. Note that, glass block was maintained at 600° C. for 1hour.

Also, the above glass block according to the optical glass of No. 3 andthe optical glass of No. 4 were increased with the temperature to 570°C. from 25° C. in air taking 2 hours, and annealed at 570° C. (the heattreatment), then carried out the procedure to reduce the coloring of theglass block (the optical glass material). Then, the glass block wascooled to the usual temperature at the temperature decreasing speed of−30° C./hour. Note that, glass blocks were maintained at 570° C. for 4hours and 30 minutes.

After the annealing, βOH value, λτ80, the refractive index nd, Abbenumber vd, and the glass transition temperature of the glass block (theoptical glass) were measured. For the optical glass of No. 1 and No. 3,βOH value, T450 and λτ80 are shown in Table 10; and for the opticalglass of No. 1 to No. 4, the refractive index nd, Abbe number vd and theglass transition temperature Tg are shown in Table 1 and Table 2.

Note that, the measured values of the refractive index nd and Abbenumber vd are the value measured using the sample cooled at the coolingspeed of 30° C. per hour. For the measured value of the liquidustemperature LT, the sample was re-heated, and maintained for 2 hours,then cooled to room temperature. Then, the presence of the crystalprecipitation inside the glass was verified by the optical microscope,and the lowest temperature of which the crystal is not present was setas the liquidus temperature.

The examples 1 to 3 of Table 10 are the data regarding the optical glassproduced without introducing the water vapor in the melting containerfrom the pipe made of platinum; and the examples 4 to 6 are the dataregarding the optical glass produced by introducing the water vapor intothe melting container from the pipe made of platinum. The examples 1 to3 used orthophosphoric acid raw material, and also the air tightness ofthe melting container was enhanced, thereby the water was introducedinto the molten glass and suppressed the evaporation of the water vaporfrom the melting container. Further, for the examples 4 to 6, the watervapor partial pressure in the melting container was actively increased.

When T450 and λτ80 of the optical glass of the examples 1 to 3, and T450and λτ80 of the optical glass of the examples 4 to 6 were compared, theexamples 4 to 6 wherein the water vapor partial pressure inside themelting container was actively increased had larger βOH value as well;and it can be understood that even more significant reduction of thecoloring degree has been done. As such, the optical glass having thecomposition of No. 1 shown in Table 1 and No. 3 of Table 2 having littlecoloring due to the heat treatment can be obtained.

TABLE 10 Flow amount of βOH T450 λ80 the water vapor [mm⁻¹] [%] [nm][litter/min] No. 1 No. 3 No. 1 No. 3 No. 1 No. 3 Example 1 0 0.77 0.1467.75 14.25 502 — Example 2 0 0.52 0.17 58.06 15.42 — — Example 3 0 0.380.21 51.96 25.6 — — Example 4 2 1.53 0.97 77.99 64.76 423 457 Example 52 1.62 1.17 73.35 68.2 417 450 Example 6 2 1.69 1.17 76.50 65.21 422 455

Note that, for the examples 1 to 6, even if the optical glass to beproduced is changed from the optical glass composition of No. 1 shown inTable 1 to the optical glass composition shown in Table 2, or even if itis changed from the optical glass composition of No. 3 to the opticalglass composition of No. 4, the coloring degree can be made smallersignificantly. Also, for the examples 1 to 6, the crucible made ofplatinum was used as the melting container, however the optical glasshaving significantly smaller coloring degree can be obtained by usingthe crucible made of platinum alloy, gold, gold alloy or so as themelting container to produce the optical glass, then carrying out theheat treatment to the obtained optical glass. Further, for the examples4 to 6, the water vapor was supplied into the platinum crucible coveredwith lid via the pipe, however the same effect can be obtained bybubbling the water vapor into the molten glass in the platinum crucible.This is same even when the optical glass to be produced is changed tothe composition of No. 2 shown in Table 1 and the composition of No. 4shown in Table 4.

Also, in the examples 4 to 6, as the water vapor supplied into thecrucible made of platinum, the water vapor obtained by boiling the waterusing the boiler was used. However, when producing the optical glassmaterial, the water vapor obtained by other method can be usedaccordingly. For example, the water sprayed in a mist form to the glassmelting furnace of flame resistant which houses the melting containersuch as crucible made of platinum or so to make the water vapor, thenwater vapor partial pressure of the atmosphere inside the glass meltingfurnace and the melting container may be increased. Alternatively, thewater may be supplied into the glass melting furnace using the pump, andboiling the water by the heat inside the melting furnace, therebyforming the water vapor and the water vapor partial pressure in theglass melting atmosphere may be increased; or other method may be usedas well. The water content in the optical glass material can beincreased by using these methods.

Comparative Example 1

The glass block (the optical glass material) was produced as same as theexamples 1 to 3 except that it was maintained opened by removing the lidmade of platinum, then the heat treatment was carried out as same as theexamples 1 to 6. However, the coloring degrees of the glass block (theoptical glass) being heat treated was larger than the examples 1 to 6.

Also, the glass block (the optical glass material) was produced as sameas the comparative example 1 except that the glass composition was theglass composition of No. 2 and No. 4 instead of No. 1 and No. 3, and theheat treatment was carried out. However, the coloring degrees of theglass block the optical glass material) being heat treated was largerthan the examples 1 to 6.

Comparative Example 2

Except for introducing nitrogen gas instead of water vapor into themelting container, the glass block (the optical glass material) wasproduced as same as the examples 4 to 6, and the heat treatment wascarried out as same as the examples 1 to 6. The coloring degree of theglass block (the optical glass) being heat treated was significantlylarger than the glass block (the optical glass) of the comparativeexample 1.

Comparative Example 3

The glass block (the optical glass material) was produced as same as theexamples 4 to 6 except that the reducing gas was introduced into themelting furnace instead of the water vapor, then the heat treatment wascarried out as same as the examples 1 to 6. However, the coloring degreeof the glass block being heat treated (the optical glass) was extremelylarger than the glass block (the optical glass) of the comparativeexample 1.

Note that, if the concentration of the reducing gas is high, thereducing gas component forms alloy with the platinum crucible, andcauses to break the crucible. This is same for the cases when the glasscomposition is changed to the composition of No. 2 shown in Table 1 andNo. 4 shown in Table 2.

(The Detail of the Observation Result of the Coloring Degree of theGlass Block at Before and after the Heat Treatment)

The observation results of the coloring degree of before and after theheat treatment of the glass block produced in the examples and thecomparative examples are shown in Table 11. Note that, the coloringdegree was evaluated by placing the glass block having planar shape ofapproximate circular shape on the white paper and visually observingunder the room light. Note that, the glass block of the examples and thecomparative examples used for the observation had approximately the samethickness. Also, the evaluation standard of the transparency shown inTable 11 is as follows. A: although the glass block (the optical glass)is lightly colored, it is clear enough to recognize the whiteness of thepaper positioned below the glass block (the optical glass) (Hightransparency). B: although the glass block (the optical glass) iscolored, it is clear enough to recognize the paper positioned below theglass block (the optical glass) (moderate transparency). C: the glassblock (the optical glass) is heavily colored, and it has lowtransparency such that the paper positioned below the glass block (theoptical glass) can be barely recognized (low transparency). D: the glassblock (the optical glass) is completely opaque, and the paper positionedbelow the glass block (the optical glass) cannot be recognized (opaque).

TABLE 11 Composition; No. 1 and No. 2 Composition; No. 3 and No. 4Before heat treatment After heat Before heat treatment After heat (rightafter the molding) treatment (right after the molding) treatment ColorClarity Color Clarity Color Clarity Color Clarity Example 1 to 3 darkbrown D Yellow A black D yellow A Example 4 to 6 dark brown D lightyellow A black D light yellow A Comparative brown C brown B dark purpleC purple B example 1 Comparative black D dark brown D black D darkpurple D example 2(The Verification of Platinum being Mixed)

Among the glass blocks of after the heat treatment which were used inthe examples 1 to 6 and the comparative examples 1 to 3, the inside ofthe glass blocks were observed by the optical microscope except forthose having “D” for the transparency evaluation. As a result, noplatinum contaminant being mixed or the crystal precipitated was foundin the inside of all the glass blocks. Also, the platinum dissolvedamount in the glass block used in the examples the examples 1 to 6 andthe comparative examples 1 to 3 were measured by ICP spectrometry, theresults were less than 2 ppm for all cases.

Example 7

The optical glass produced in the examples 1 to 6 were processed into aglass material for press-molding, then heating, softening and apress-molding were carried out, thereby the optical element blank wasproduced. Further, the optical element blank was processed and theoptical element such as spherical lens and prisms or so was produced.Further, to the lens surface or the prism surface, the anti-reflectionfilm was coated; thereby the final product was obtained. For the opticalglass of No. 2 shown in Table 1 and No. 4 shown in Table 2, the glassmaterial for press-molding, the optical element blank and the opticalelement were produced as same.

Hereinafter is the summary regarding the main embodiments and variousmodified examples.

The glass of the embodiment according to the first modified example hasthe refractive index of 1.75 or more, and βOH value shown in belowequation (1) satisfies the below equation (6).

BOH=−[ln(B/A)]/t  (1)

βOH≧181.39×nd ⁻³−325.75×nd ⁻²+194.85×−38.1  (6)

[In the equation (1), “t” is a thickness of said glass used for ameasurement of an external transmittance, “A” is the externaltransmittance (%) at a wavelength of 2500 nm when a light enters intosaid glass in parallel to a thickness direction thereof, and “B” is sthe external transmittance (%) at the wavelength of 2900 nm when a lightenters into said glass in parallel to the thickness direction thereof.Also, “ln” is natural logarithm. In the equation (6), “nd” is therefractive index of said glass.] Note that, the unit of βOH is mm⁻¹.

The preferable glass of the embodiment according to the first modifiedexample has the content of the noble metal of 4 ppm or less in theglass.

The preferable glass of the embodiment according to the first modifiedexample includes P₂O₅ as said glass component.

The preferable glass of the first embodiment according to the secondmodified example has the refractive index nd of 1.9 or more and lessthan 1.97, and

it is the oxide glass comprising at least one oxide selected from thegroup consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as a glass component,wherein

a total content of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the rangeof 30 mol % to 60 mol %, and

βOH value shown in below equation (1) is 0.1 mm⁻¹ or more.

βOH=−[ln(B/A)]/t  (1)

[In the equation (1), “t” is a thickness of said glass used for ameasurement of an external transmittance, “A” is the externaltransmittance (%) at a wavelength of 2500 nm when a light enters intosaid glass in parallel to a thickness direction thereof, and “B” is sthe external transmittance (%) at the wavelength of 2900 nm when a lightenters into said glass in parallel to the thickness direction thereof.Also, “In” is natural logarithm.]

The preferable glass of the first embodiment according to the secondmodified example includes P₂O₅ as said glass component within the rangeof 15 mol % to 35 mol %.

The preferable glass of the second embodiment according to the secondmodified example has the refractive index rid of 1.97 or more, and

it is the oxide glass comprising at least one oxide selected from thegroup consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as a glass component,wherein

a total content of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is within the rangeof 40 mol % to 80 mol %, and

βOH value shown in below equation (1) is 0.1 mm⁻¹ or more.

βOH=−[ln(B/A)]/t  (1)

[In the equation (1), “t” is a thickness of said glass used for ameasurement of an external transmittance, “A” is the externaltransmittance (%) at a wavelength of 2500 nm when a light enters intosaid glass in parallel to a thickness direction thereof, and “B” is sthe external transmittance (%) at the wavelength of 2900 nm when a lightenters into said glass in parallel to the thickness direction thereof.Also, “ln” is natural logarithm.]

The preferable glass of the second embodiment according to the secondmodified example includes P₂O₅ as said glass component within the rangeof 10 mol % to 35 mol %.

The preferable glass of the first and the second embodiment according tothe second modified example satisfies the below equation (11).

λτ80<aX+b  (11)

[In the equation (11), λτ80 refers to the wavelength (nm) wherein theinternal transmittance (internal transmittance τ) is 80% which iscalculated first by measuring the internal transmittance at the range ofthe wavelength 280 to 700 nm when the light enter into the optical glassparallel to the thickness direction thereof, then assuming that thethickness of the optical glass based on the internal transmittancemeasured thereby is 10 mm. “a” is the constant (1.8359 nm/mol %), “b” isthe constant (351.06 nm), and X is a total content (mol %) of said TiO₂,Nb₂O₅, WO₃ and Bi₂O₃]

The preferable glass of the first and the second embodiment according tothe second modified example includes the content of the antimony oxideof less than 1000 ppm in terms of Sb₂O₃.

Further, preferable glass of the main embodiment and the above mentionedmodified example is a glass comprising 25 mol % or more of a totalcontent of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃, more preferably of 30 mol %or more, and further preferably of 35 mol % or more.

The preferable glass of the main embodiment and the above mentionedmodified example is a glass wherein the content of P₂O₅ is larger thanthe content of SiO₂ in mol % expression.

The preferable glass of the main embodiment and the above mentionedmodified example is a glass wherein the content of P₂O₅ is larger thanthe content of B₂O₃ in mol % expression.

The preferable glass of the main embodiment and the above mentionedmodified example is a glass wherein the content of P₂O₅ is larger thanthe total content of SiO₂ and B₂O₃ in mol % expression.

The preferable glass of the main embodiment and the above mentionedmodified example is a glass wherein the content of P₂O₅ is 10 mol % ormore.

The preferable glass of the main embodiment and the above mentionedmodified example is a glass wherein the content of P₂O₅ is 40 mol % orless.

The preferable glass of the main embodiment and the above mentionedmodified example has a content of GeO₂ of 0 to 10 mol %, more preferablyof 0 to 5 mol %, further preferably of 0 to 3 mol %, even morepreferably of 0 to 2 mol %, even further preferably of 0 to 1 mol %, andeven furthermore preferably of 0 to 0.5 mol %.

The preferable glass of the main embodiment and the above mentionedmodified example has a content of TeO₂ of 0 to 10 mol %, more preferablyof 0 to 5 mol %, further preferably of 0 to 3 mol %, even morepreferably of 0 to 2 mol %, even further preferably of 0 to 1 mol %, andeven furthermore preferably of 0 to 0.5 mol %.

The preferable glass of the main embodiment and the above mentionedmodified example has the content of Sb₂O₃ of 0 ppm or more and less than1000 ppm; and further preferable glass has the content of Sb₂O₃ of 900ppm or less, more preferably 800 ppm or less, and even more preferablyof 700 ppm or less, even further preferably of 600 ppm or less, stillmore preferably of 500 ppm or less, and it is even preferable in theorder of 400 ppm, 300 ppm, 200 ppm, 100 ppm.

The preferable glass of the main embodiment and the above mentionedmodified example has the total amount of P₂O₅, SiO₂, B₂O₃, TiO₂, Nb₂O₅,WO₃, Bi₂O₃, MgO, CaO, SrO, BaO, ZnO, Li₂O, Na₂O, K₂O, Al₂O₃, ZrO₂, GeO₂,TeO₂ and Sb₂O₃ of preferably 90% or more, more preferably 92% or more,further preferably 95% or more, even more preferably 96% or more, evenfurther preferably 97% or more, still more preferably 98% or more, andyet more preferably more than 99%.

The glass of the main embodiment and the above mentioned modifiedexample is preferably substantially be free of Pb, As, Cd, U, Th and Tlfrom the point of reducing the environmental load.

The glass of the main embodiment and the above mentioned modifiedexample is preferably substantially free of the additives and thecomponents which have absorbance in the visible range such as Cu, Cr,Mn, Fe, Co, Ni, V, Mo, Nd, Eu, Er, Tb, Ho, Pr or so.

The preferable glass of the main embodiment and the above mentionedmodified example has the content of the noble metal of 4 ppm or less inthe obtained glass. The preferable upper limit of the noble metalincluded in the glass is 3 ppm, 2.7 ppm, 2.5 ppm, 2.2 ppm, 2.0 ppm, 1.8ppm, 1.6 ppm, 1.4 ppm, 1.2 ppm, 1.1 ppm, 1.0 ppm, 0.9 ppm; and the lowerthe upper limit is the more preferable it is.

The preferable glass of the main embodiment and the above mentionedmodified example has the content of Pt of 4 ppm or less in the obtainedglass. The preferable upper limit of the noble metal included in theglass is 3 ppm, 2.7 ppm, 2.5 ppm, 2.2 ppm, 2.0 ppm, 1.8 ppm, 1.6 ppm,1.4 ppm, 1.2 ppm, 1.1 ppm, 1.0 ppm, 0.9 ppm; and the lower the upperlimit is the more preferable it is.

The preferable glass of the main embodiment and the above mentionedmodified example has the refractive index nd of 1.75 or more, morepreferably 1.80 or more, further preferably of 1.85 or more, and evenmore preferably of 1.90 ormore.

The preferable glass of the main embodiment and the above mentionedmodified example is an optical glass.

1. An optical glass comprising at least one oxide selected from thegroup consisting of TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ as a glass component,wherein a total content of said TiO₂, Nb₂O₅, WO₃, and Bi₂O₃ is 20 mol %or more, and a value of βOH shown in below equation (1) satisfies arelation shown in below equation (2);βOH=−[ln(B/A)]/t  (1);βOH≧0.4891×ln(1/HR)+2.48  (2); in the equation (1), “t” is a thicknessof said glass used for a measurement of an external transmittance, “A”is the external transmittance (%) at a wavelength of 2500 nm when alight enters into said glass in parallel to a thickness directionthereof, and “B” is s the external transmittance (%) at the wavelengthof 2900 nm when a light enters into said glass in parallel to thethickness direction thereof. In the equation (2), “HR” shows a totalamount (mol %) of a content of each component of TiO₂, Nb₂O₅, WO₃ andBi₂O₃ in said glass. In the equations (1) and (2), “ln” is naturallogarithm.
 2. The optical glass as set forth in claim 1 comprising P₂O₅as said glass component.
 3. The optical glass as set forth in claim 2,wherein a content of P₂O₅ is larger than a content of B₂O₃ in terms ofmol % expression.
 4. The optical glass as set forth in claim 1, whereina total content of said TiO₂, Nb₂O₅, WO₃ and Bi₂O₃ is 25 mol % or more.5. The optical glass as set forth in claim 1 wherein substantially freeof V.
 6. A glass material for press-molding comprising the optical glassas set forth in claim
 1. 7. An optical element comprising the opticalglass as set forth in claim 1.